GIF needed
Up-tilt has an insane hitbox and is very rewarding to hit. All this power is locked behind one little issue: it all happens behind Fox. Therefore, you have to turn your back to the opponent without dashing back, tap jumping or hitting up-angled forward-tilt instead.
While it requires a fair amount of precision with the control stick, spacies' up-tilts are some of the best in the game, and it would be foolish not to abuse them to the fullest extent.