Perfect-shielding is a defense mechanic that rewards precise shield-release timing. The moment you release the Shield button, there’s a five-frame window where your shield is still active. If your fighter blocks an attack during this window, they block the attack while negating all shield-stun. This lets them act again after dropping shield much faster than normal, usually early enough to allow guaranteed punishment.
To register a perfect shield parry, a raised shield must be up for at least three frames. This means perfect shield isn’t possible after a shield flickers on for only two frames. The fastest possible perfect shield block occurs in four frames, tapping and releasing the Shield button as though for a short hop. If you quickly tap and release the Shield button, the earliest perfect shield block is four frames later (three frames of a normal raised shield, then five frames with a chance to perfect shield).