The rapid effect (known internally as rush) is used by a small amount of attacks in Super Smash Bros. Ultimate, mainly those that strike quickly and repeatedly. The effect has a subtle aesthetic difference from the normal effect, in that the random displacement of the visuals when hitting an opponent is larger, with a maximum distance of 7 units from the collision point instead of 2 units. Otherwise, both effects function identically.
List of attacks with rapid hitboxes