How to perform
The player must choose Marth or Roy in a Vs. match and choose Dreamland (64) as the stage. Then, on the last 2 frames of Whispy blinking, the player must execute a jab, Forward tilt, up tilt, down tilt, dash attack, or standing grab.
The hitbox for thase attacks (sans standing grab) will disappear until the move finishes.
As for the standing grab, Marth/Roy does the grab without lunging forward, making it safer than a normal grab.
Explanation
No one knows why the hitbox dissappears when Whispy blinks.