From SmashWiki, the Super Smash Bros. wiki
The pseudo upsmash is used by
Ness mainly in
Super Smash Bros Melee. Due to the lack of knockback and low damages of Ness' upsmash, many pro players have found ways around it by using the pseudo upsmash. This involves shffling the up aerial so that the hit box is low enough to the ground so that is appears as though Ness never left the ground but instead has just done an upsmash attack. There are many applications of this because it is able to be done while dashing, wavedashing and just about any movement in the game.