Smash directional influence
Smash directional influence (commonly shortened to Smash DI or SDI, and officially known as Hitstun Shuffling since Super Smash Bros. 4) is a mechanic that allows players to slightly alter their position during the freeze frames of being hit by an attack. Tapping the control stick (and/or C-stick in Super Smash Bros. Brawl) in any direction during freeze frames 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.
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 in a given direction every frame a new directional vector of sufficient size is input. 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 are considered to be two vectors, 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 Captain Jack 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 SDI horizontally, not vertically. This prevents them from SDIing upward and going airborne before getting launched by the attack, thus ensuring they remain on the ground.
As SDI is performed during freeze frames, a move is easier to SDI the more freeze frames it inflicts, and essentially impossible to SDI if it has no freeze frames. This is also why the mechanic is more effective against multi-hit moves, as each hit puts the opponent in freeze frames and thus extends the total amount.
Variants of SDI
Automatic smash directional influence
Automatic smash directional influence (ASDI for short) is a weaker variant of SDI. Once a character's freeze frames are 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, as the downward shift from holding the control stick down allows characters to land on the ground earlier in addition to taking reduced knockback from the crouch cancel, and thus retaliate against attacks much quicker than usual.
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 freeze frames (and thus before shieldstun), players can tap or hold a direction on the control stick to shift along the ground while holding shield, with each shield SDI pulse covering 2/3 of the distance of a regular SDI pulse. This is especially useful to escape some grounded multi-hit attacks, most notably Peach's down smash in Melee.
Effectiveness across games
In Smash 64, Melee and Brawl, SDI is very effective, especially in the latter due to attacks possessing longer freeze frames. In these games, every SDI pulse moves a character 6 units in the direction chosen, or 3 units with ASDI. 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.
Beginning in Brawl, all attacks have an SDI multiplier attached to them, which affects how far opponents can shift via SDI 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 Zelda'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 freeze frames 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 Marth's forward smash. Conversely, some multi-hit moves have higher multipliers, such as the linking hits of Mario's down aerial and Pit's neutral aerial having multipliers of 1.5× and 1.4× (respectively), allowing characters to SDI out of them even more easily.
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 drastic nerfs to SDI in Smash 4, there are extremely few attacks against which it is particularly effective, namely Ryu's light up tilt, and most infamously Bayonetta'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 freeze frames, and ability to otherwise lead into guaranteed combos that can both rack up significant damage and KO. In other cases, it is generally considered useless due to its negligible effect without an exorbitant amount of SDI pulses, and disregarded in favor of DI whenever possible. Likewise, ASDI and shield SDI have lost all 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.
In Super Smash Bros. Ultimate, SDI retains its shorter distance from Smash 4, and has been further limited. Characters can only SDI by one pulse every 4 freeze frames, and if any SDI inputs occur during these frames, the latest one will be 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 considered very minor in standard play, only having a major effect in high-leveled CPU players. Automatic SDI is also no longer affected by SDI multipliers. Additionally, due to the severe nerfs to Bayonetta and Ryu from Smash 4 (especially the former), who were the main characters SDI was impactful against, the technique is generally even less useful; as of the current metagame, its only notable use is to improve the chances of escaping Pichu's "lightning loops", which involve continuously dragging opponents down with the setup hits of its back aerial and launching them back up with its up tilt.