Shieldstun (referred to as Downtime from Shielding by Super Smash Bros. Ultimate's tips and official patch notes) is an inactionable period that occurs when a character's shield is hit. It is comparable to hitstun, in that it occurs after hitlag and only for the defending character. While the attacker still experiences the hitlag of the move, they do not undergo shieldstun, allowing them to move again before their opponent can if the ending lag of the attack is low. During the period of shieldstun, the shielding player cannot perform any of their normal out of shield options, rendering them stuck. Several fast multi-hitting moves (such as Fox's or Yoshi's down aerials) hit fast enough to keep an opponent in shieldstun for the duration of the move, as each hit lands while the opponent is still in shieldstun from the previous hit; this is usually referred to as a "blockstring", a term stemming from similar situations with block mechanics in traditional fighting games. In addition to not being able to perform out of shield options, players in shield stun cannot release the shield, adjust the tilt of their shield, shield drop, or change their shield's density. Attempting to perform any of these actions while in shieldstun will have them happen the frame shieldstun ends instead.
With the inability to jump, grab, roll, or spot dodge out of their shield, the player in shieldstun is very susceptible to another hit or a grab. Though it is rare, this can sometimes be used to the point of breaking a player's shield, as with Fox's and Falco's pillaring combos. A combo meant to lock an opponent in shield until their shield breaks is known as a shield break combo. However, outside of SSB, these combos are not perfect, as all other combos on shield have gaps in shieldstun. Thus, the goal of shield combos in Melee is rarely to outright break shields: it is far more likely an attempt at causing a shield poke, or to bait and punish a roll, grab, or spot dodge. Still, since gaps in shieldstun must exist it is necessary to mixup where and for how long these gaps fall. Doing so covers different out of shield options that the defending player might want to use. As an example, a double shine covers the grab-sized gap that is normally left after a waveshine or a jump out of shine into an aerial, and grants the attacker the hit they were looking for.
In Smash 64, shieldstun is extremely high. When hit on shield, the majority of moves leave the defending player in such a long period of shieldstun, that another hit is nearly always guaranteed. This is especially apparent with aerials with the use of Z-cancelling. This, along with the lack of perfect shielding, makes shield break combos easier and far more frequent. Some moves, such as Kirby's up tilt, are notorious for breaking shields on their own with repeated use.
The formula for shieldstun frames in Smash 64 is
In Melee, shieldstun duration in general was reduced. A large difference from other Smash games is the introduction of analog shielding. The analog shield level used when blocking affects shieldstun: the lighter the shield, the more shieldstun the blocker suffers. The formula for shieldstun frames in Melee is
This amount of shieldstun, while heavily reduced, is still sufficient to leave smart and quick attackers at an advantage. Many moves are unsafe on shield, and it is possible for defending players to capitalize on such hits. But many moves, especially certain aerials, are still safe. Pressuring an opponent's shield is challenging and risky, but highly rewarding. Often, despite a move merely breaking even on shield, or even being a few frames negative, the attacker will still have an advantage. Options out of shield, especially movement, are so limited that good spacing or quick dash dancing out of lag allow the attacker to safely pressure despite frame disadvantage.
While Melee generally has no input buffering, it is possible to buffer out of shield options by holding the C-stick during shieldstun. The character will then jump, roll, or spot dodge on the first frame after they leave shieldstun, depending on the direction held.
Yoshi is a notable exception to these rules. While he cannot jump out of shield in this iteration outside digital shield startup, he suffers minimal or no shieldstun, which combined with the other unique properties of his shield, gives his shield game a very different dynamic to the rest of the cast. Yoshi only receives shieldstun when blocking during any of his first 4 shield startup frames, and in those cases suffers an amount of shieldstun equal to
In Brawl, the formula for shieldstun frames is
Yoshi's shield is no longer immune to shieldstun, and unlike the rest of the cast in the transition from Melee, his shield still takes 15 frames to drop. In combination with him having one of the slowest shield grabs, as well as still being unable to jump out of shield, this gives him significantly worse defensive options than any other character.
In Smash 4, shieldstun was originally thought to have been reduced again from Brawl, but further testing showed this to be false. The formula for shieldstun frames prior to version 1.1.1 was
However, following version 1.1.1, shieldstun from direct attacks was increased dramatically, to the point of being higher than in Melee, and received further specialization for various cases. The new formula is
As a result of this formula, all attacks once again inflict shieldstun regardless of their damage output, and direct attacks are especially safer, to the point of various landing aerial attacks only being a few frames negative on shield with proper timing. In turn, perfect shielding having an effect on shieldstun ties further into its purpose of being much more precise, yet much more rewarding than normal shielding. Compared to previous versions of Smash 4, these changes allowed several characters to play more offensively, though due to out of shield options still being relatively fast and highly rewarding, as well as the aforementioned damage/lag nerfs, the game remained generally more defensive than Melee. On the other hand, the specific multiplier for projectiles made them generally safer as well, but to a lesser extent and only up to high damage values, preventing powerful ones such as Charge Shot from becoming too difficult to punish.
Version 1.1.3 of the game added an additional wrinkle to shieldstun: if a character is trapped in shieldstun for at least 10 hits (or in more colloquial terms, a blockstring), they are allowed to cancel it with a roll or spot dodge, and the intangibility of either option comes out immediately on frame 1, regardless of their coded startup. Practically, very few moves can hit a shield enough times for this to trigger; the looping hit of a charging Aura Sphere is the most noticeable. In the case of said move, this mechanic allows shielding characters to escape it if they are trapped by it at the ledge, where they would otherwise be forced to shield SDI or have their shield broken.
Shieldstun has been adjusted further in Ultimate, with more specific cases introduced than in Smash 4. The new formula is
As a result, shieldstun remains unchanged for smash attacks compared to Smash 4, but is increased for direct special moves and all other ground attacks; tilt attacks in particular are noticeably safer on shield. Conversely, it is decreased for aerial attacks and projectiles, most significantly the former. Further adjustments in Ultimate include short hopped aerial attacks having a 0.85× damage multiplier, further reducing the shieldstun of some of them (albeit slightly), and stale-move negation applying on shield, weakening repeated pressure with a particular attack. However, due to the universally lower landing lag, the general safety of aerial attacks on shield remains unchanged, with some of them being slightly safer. Furthermore, shield dropping takes longer (11 frames instead of 7), while shield grabbing after shieldstun has increased startup by 4 frames. Overall, attacking shields in Ultimate is more favorable, with options to safely punish out of shield being riskier (albeit still highly rewarding), promoting more offensive play.
The ability introduced in version 1.1.3 of Smash 4 to dodge-cancel blockstrings of at least 10 hits returns as well, and is now applicable to several other moves, most remarkably every rapid jab, due to their increased shieldstun and much faster hit rate. This renders it vastly more useful for punishing such moves, as in the case of rapid jabs, the shielding character can usually roll behind the attacker, who will then be forced to unleash their rapid jab's finisher and become much more vulnerable. This mechanic's larger notoriety has led to it being referred to as shieldstun canceling by the community.
Version 9.0.0 made other smaller adjustments to shieldstun:
List of moves with shieldstun multipliers
Due to a glitch, even if a move has a shieldstun multiplier only coded for specific hitboxes, the game applies the multiplier to all hitboxes of the move during frames where the specified hitbox is present. Likewise, if a move has different shieldstun multipliers for each hitbox, the multiplier for the hitbox with the highest ID number applies to all hitboxes, and other multipliers are ignored. This is noticeable for Areadbhar, where the sweetspot's shieldstun multiplier applies to the entire move for all except the grounded version's first active frame (where the sweetspot isn't present), and Kazuya's forward smash, which has a 0.1× multiplier for the sourspot and a 1.75× multiplier for the sweetspot, but the latter applies to both.