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.