Shieldstun 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. It is possible, however, to buffer an out of shield action using the C-Stick while in shieldstun: tilting the C-Stick up will buffer a jump, tilting to the side will buffer a roll, and tilting down will buffer a spot dodge. The action will execute on the first frame possible: the frame shieldstun ends. Many multiple hit moves that hit rapidly (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. 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 spotdodge. 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.
Shieldstun between games
In SSB, shieldstun is extremely high. When hit on shield, the majority of moves (especially aerials) leave the defending player in such a long period of shieldstun, that another hit is nearly always guaranteed. 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 frames of shieldstun in SSB is: (X * 1.62) + 4 (rounded down), where X is the amount of damage the attack would deal if it was unshielded (shield damage dealt by the move is not factored into this formula).
In Melee, shieldstun duration in general was reduced. The formula for frames of shieldstun in Melee is (X + 4.45) / 2.235 rounded down, where X is the amount of damage the attack would deal if it was unshielded (shield damage dealt by the move is not factored into this formula). 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.
In Brawl, shieldstun was further reduced. Very few (if any) attacks on shield are safe. Combined with the lowered time to drop shield, this means that hitting shields in Brawl puts attackers in a very vulnerable position. The approximate formula for frames of shieldstun in Brawl is: X * .2875 + 2 rounded down (shield damage dealt by the move is not factored into this formula).
In Smash 4, shieldstun was originally thought to have been reduced again from Brawl, although further testing has since shown this to be false. The working formula for frames of shieldstun in Smash 4 prior to patch 1.1.1 was originally: X / 2.56 rounded down (shield damage dealt by the move is not factored into this formula), which results in barely higher shieldstun compared to Brawl .
However, following the aforementioned patch, shieldstun from non-projectile attacks was increased dramatically, to the point of being higher than in Melee. The new formula is: (X / 1.75) + 2 rounded down. For projectiles, however, the formula is now (X / 3.5) + 2, giving powerful projectiles less shieldstun, while giving weaker ones more.
Although the increased shieldstun makes attacks safer on shield than in the game's initial release, attacks in Smash 4 tend to have less favorable autocancel windows and push shielding characters a shorter distance than in Brawl, reducing their safety; these factors may have been responsible for many thinking that shieldstun had been decreased prior to the patch.