Smash directional influence

Smash directional influence (commonly shortened to Smash DI or SDI, and officially known as  since Super Smash Bros. 4) is a mechanic that allows players to slightly alter their position during hitlag from being hit by an attack. Tapping the control stick (and/or C-stick in Super Smash Bros. Brawl) in any direction during the 2nd hitlag frame and onward will slightly move their character in that direction, allowing them to potentially escape multi-hit moves or certain combos. SDI can also be used to move into a wall or ceiling in order to tech near-instantly or to (very minimally) increase the distance from the blast line that the attack is about to send them towards.

General properties
Despite its name, SDI is not a subtype of directional influence (or DI), but a different mechanic altogether that can be performed under the same circumstances. The major difference is that, while DI changes the trajectory a character is launched in, SDI changes a character's position before launch. DI does not exist in the original Super Smash Bros., while SDI does; as a result, SDI in Super Smash Bros. is often referred to as simply "DI".

The mechanics of SDI operate by shifting the character by a vector determined by the control stick. SDI is applied on every frame a control stick vector of sufficient length is input, in a new direction on either horizontal or vertical axis. As a result, repeatedly tapping in one direction will move the character much farther than simply holding the stick in that direction. In addition, because diagonals have both horizontal and vertical components, changing from an orthogonal position to an adjacent diagonal one will generate a new SDI input, while changing from a diagonal position to an adjacent orthogonal one will not. This results in a technique called quarter-circle DI, which involves quickly rotating the stick between two orthogonal positions for increased distance. Quarter-circle DI was introduced to the United States by Japanese smasher and was once known as Japanese DI.

In Brawl, the C-stick can assist in SDI. So-called double-stick SDI utilizes the C-stick's ability to override the control stick by holding one of the sticks in one direction and utilizing quarter-stick DI with the other.

Whenever a character is hit on the ground by an attack that does not lift them off it, such as a weak meteor smash or move that launches at the Sakurai angle, they can only leave ground horizontally, not vertically. This prevents them from SDIing upward and going airborne before getting launched by the attack, potentially allowing them to immediately land and cancel hitstun. When grounded, the horizontal component of a diagonal SDI vector still moves the character, only vertical component is negated. Consequently, quarter circle SDI and other methods to perform multiple SDI inputs still work. In Ultimate when hit with a move that will lift the victim off the ground, it is possible to SDI upwards only if the first SDI input is made on exactly frame 2 of hitlag, and it is in an upwards direction. Doing so lifts the opponent off the ground during hitlag, allowing for further upwards SDI inputs to be made. Notably, this allows opponents to jump out of fast hitting moves such as 's down tilt which would otherwise force a landing during hitstun, before hitting the ground, as they will be launched from higher up, and thus the hitstun will end before they land. However, due to the frame perfect timing required, which also demands a correct prediction of what move will hit the victim and on what frame, this has little to no utility in realistic circumstances.

As SDI is performed during hitlag, a move is easier to SDI the more hitlag it inflicts, and impossible to SDI if it has less than two frames of hitlag. This is also why the mechanic is more effective against multi-hit moves, as each hit puts the opponent in hitlag and thus extends the total amount.

Exceptional displays of SDI are known by some as "Wizzy SDI,” (in Melee) or “Doom SDI" (in Brawl), named after Wizzrobe and Mr. Doom respectively, who have been renowned for their ability to repeatedly perform TAS-like SDI in matches. Wizzrobe's method of SDI is most commonly referred to as wank DI, this method dates back to Smash 64 as there is no DI mechanic in the game, only SDI, so players had to develop the wank DI method to more easily escape combos. The top-level smasher, aMSa informed Wizzrobe that wank DI was also in Melee, and taught him how to do it. In order to wank DI, instead of moving the control stick with one's thumb, the player must change their grip and hold the controller with one hand while pressing their thumb against the control stick with the other hand and then shake the controller in a sort of 'wanking' motion, hence the name.

Automatic smash directional influence
Automatic smash directional influence (ASDI for short) is a weaker variant of SDI. Once a character's hitlag is over, they shift slightly in whatever direction the control stick is currently held, by a shorter distance than an SDI input. If the C-stick is also being held in a direction, it will override the control stick's input, allowing the player to DI in another direction simultaneously. ASDI is significantly less useful than regular SDI and is generally ignored, as any stick position during this time is likely to be attempted at regular DI. In Super Smash Bros. Melee, however, it is an important factor in crouch canceling. It is often used in place of crouch cancelling against moves that have the Sakurai Angle, such as the weak hit of Marth's Dolphin Slash, as at specific percents they are impossible to crouch cancel. The downward shift from ASDI down may allow a character to land on the ground immediately. If the character wasn't knocked down, the landing cancels hitstun, and thus allows them to retaliate against attacks much quicker than usual. It is additionally key in utilizing techniques like Amsah techs and Slideoff DI, which use ASDI to create tech scenarios and escape combos respectively.

Shield smash directional influence
Similar to SDI, shield SDI (officially named Shieldstun Shuffling by the tips in Smash 4 and Ultimate) is a mechanic that allows players to move slightly when their shield is hit. It is nearly identical to SDI in its function and how it is performed: during hitlag (and thus before shieldstun), players can tap or hold a direction on the control stick to shift along the ground, with each shield SDI pulse covering 2/3 (or exactly 0.66× in Melee) of the distance of a regular SDI pulse. However, contrary to SDI, shield SDI can only be applied horizontally, and vertical components of control stick inputs are completely ignored. Thus, techniques such the quarter-circle SDI wont result in multiple shield SDI inputs. Shield SDI is especially useful to escape some grounded multi-hit attacks, most notably Peach's down smash in Melee.

There is also shield ASDI, which has the already familiar relation to shield SDI: It's half as effective as shield SDI, and is applied on first frame of shield recoil, after hitlag.

Ceiling SDI
In Ultimate, when colliding against a surface with sufficient launch speed, the player will be unable to tech. However, when colliding with a floor or ceiling, it is possible to shift slightly to the left or right by holding in that direction when colliding.

Super Smash Bros. and Super Smash Bros. Melee
In Smash 64 and Melee, SDI is highly useful, especially in the former game due to its higher hitlag and lack of other defensive options (such as DI and air dodging). In these games, every SDI pulse moves a character 6 units in the direction chosen, or 3 units with ASDI in Melee. With enough pulses, SDI allows characters to escape a majority of multi-hit moves before they can connect their final, usually stronger hits; as a result, multi-hit moves with weak linking hits and where the character remains stationary, such as rapid jabs and Samus' up smash, are regarded as ineffective due to their poor reward and extreme vulnerability if they are escaped, while others that allow movement during their use, such as Fox and Jigglypuff's down aerials, require careful spacing of their hits to either land all of them or get the opportunity to punish opponents after they SDI out.

Super Smash Bros. Brawl
SDI remains particularly effective in Brawl, which like Smash 64, has higher hitlag compared to Melee. While it retains the same base distance as in these games, one major change is that all attacks have a specific SDI multiplier, which modifies how far opponents can shift in a direction with each SDI pulse when hit by them. This also affects ASDI, but not shield SDI. For most attacks, this multiplier is 1×, allowing them to be SDIed normally. However, certain multi-hit moves have lower multipliers, reducing their SDI ability to the point they can connect more reliably; examples of such moves are the linking hits of 's forward smash and up smash, which have SDI multipliers of 0.5× and 0.1× (respectively) allowing them to connect much more reliably than in Melee, where even ASDI alone was enough to escape them. Likewise, some moves with noticeably long hitlag use an SDI multiplier of 0×, effectively making them impossible to SDI, and preventing characters from shifting long distances away as they are hit; examples are the sweetspots of Zelda's Lightning Kicks, as well as the tipper of 's forward smash. Conversely, some multi-hit moves have higher multipliers, such as the linking hits of 's down aerial and 's neutral aerial having multipliers of 1.5× and 1.4× (respectively), allowing characters to SDI out of them even more easily.

Super Smash Bros. 4
In Smash 4, SDI has been significantly weakened. The distance characters can shift in a direction is now 2 units per SDI pulse (down from 6), and 1.33 units with ASDI (down from 3), which drastically reduces the distance of shield SDI as well. Additionally, many multi-hit moves with above-average SDI multipliers in Brawl, such as the aforementioned Mario's down aerial and Pit's neutral aerial, have had them reduced to normal or below-average (0.8× in the case of both moves). Lastly, double-stick SDI from Brawl is no longer possible, as SDI inputs with the C-stick are only accounted for if the control stick is in a neutral position, and vice versa. Combined with the autolink angle's noticeable improvements and much wider distribution in Smash 4, multi-hit moves are much harder to SDI out of than ever, with autolinking ones becoming impossible to escape should the player utilize them properly, thus indirectly buffing them. These changes are so drastic that players of the demo at E3 2014 initially thought SDI was removed completely, though videos later on would show that SDI did still exist, just in a much weaker state.

Because of the significant nerfs to SDI in Smash 4, there are few attacks against which it is noticeably effective, such as 's light up tilt, and most infamously 's After Burner Kick and Witch Twist beginning in update 1.1.6, due to their SDI multipliers of 2.5× and 2× (respectively), long hitlag, and ability to otherwise lead into guaranteed combos that can both rack up significant damage and KO. Despite the nerfs however, SDI can still be effective in escaping long combos, such as with the game's notorious ladder combos, as even the decreased distance of SDI can be enough to push a character out of a followup move's reach, especially after SDIing several consecutive hits. ASDI and shield SDI have lost most of their practical usefulness due to their even shorter distance, especially the latter due to not being affected by SDI multipliers that could otherwise increase its distance, though again there are situations where they can still make a difference.

Super Smash Bros. Ultimate
In Ultimate, SDI retains its shorter distance from Smash 4, and has received various other adjustments:
 * Each time an SDI input is made, the next input is not registered until 4 frames later, and if any SDI inputs occur during these frames, the latest one is buffered when another SDI pulse is possible. However, as it is difficult for human players to consistently SDI at such a fast speed, this change is generally considered minor. Notably, this limit does not apply to shield SDI, making it comparatively stronger than in Smash 4 due to the increased hitlag in Ultimate.
 * ASDI has been removed almost entirely, only applicable on moves with an electric, paralyze, or crumple effect, and a special attack effect only used for Kazuya's 10-hit combo, further limiting its application. Oddly enough, it now applies twice against moves with these effects. It also oddly still seems to apply for Final Edge.
 * For every five consecutive hits a character takes, the SDI distance for the next five hits is increased by 1.15×. As a result, the longer a character is trapped in a multi-hit attack, such as a rapid jab, the easier it is to escape with SDI.
 * If a character is hit at least 20 times, they are allowed to SDI upward even if the attack would keep them on the ground, which otherwise prevents vertical SDI, and thus go airborne. While this has very limited practical utility, it notably allows characters to escape 's "Wobbelle" setup, which involves her trapping opponents at the edge with repeated hits of her neutral attack, due to its combination of low knockback and high hitstun.
 * Since version, characters with passive shields (namely , , , and ) can also shield SDI if they block a projectile with them.

As a result, while SDI remains more limited in utility compared to the pre-Smash 4 games, it is generally more effective for escaping long sequences of multi-hit attacks than it was in Smash 4, and the general greater prominence of long combos in Ultimate has also increased its utility compared to Smash 4. Much like DI gaining a visual indicator in the form of a bright blue streak of light, SDI has gained a visual indicator manifesting itself in the form of a crescent shape angle reader when performed, allowing players to see which direction their opponent is going, though this is generally too quick for players to react to except for with moves that have extremely long hitlag, such as with ' Charge Shot.

Trivia

 * Due to SDI's notoriety as a counterplay technique against in Smash 4 (because of her ability to easily set up ladder combos that can lead to early KOs otherwise), the phrase "just SDI" has become infamous among the community as satirical advice for players who have trouble against the character. SDI in general is also commonly associated with Bayonetta as a result, to the extent that the Palutena's Guidance for Bayonetta in Ultimate references it, with Viridi noting that to escape Bayonetta's combos, players should "just[...]wiggle around or something."