Shieldstun
Shieldstun is a game mechanic that can be observed when a player's shield is hit. Upon being struck while shielding, the struck character will undergo an effect similar to that of hitstun, in addition to some freeze frames. While the attacker experiences the normal freeze frames of the move, they do not undergo shieldstun, allowing them to move again before their opponent can. During the period of shieldstun, the shielding player cannot perform any of their normal out of shield options, rendering them stuck. In Melee, the formula for frames of shieldstun is (X + 4.45)/2.235) rounded down, where X is the amount of damage the attack would deal if it was unshielded. Additionally, shield damage the move may possess is not factored into this formula.
With the inability to jump, grab, or roll out of their shield, the player in shieldstun is susceptible to another hit or a grab. Though it is rare, this can sometimes be abused to the point of breaking a player's shield, as with Fox's and Falco's pillaring combos or other shield break combo. However, it is more likely to see this used to bait out and punish a roll. 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 a each hit lands while the opponent is still in shieldstun from the last hit. These moves can lead into a grab upon landing if performed correctly.