From the Super Mario Wiki, the Mario encyclopedia
Ptooie is Piranha Plant's standard special move in Super Smash Bros. Ultimate. Hence its name, when used Piranha Plant spits a spiked ball out of its mouth and keeps it airborne by blowing on it, with the ball rising and falling periodically, similar to a Ptooie. Piranha Plant will continue to blow on the ball until it is either launched in a direction, or if the player lets to of the special button, in which case Piranha Plant swallows the ball. Piranha Plant can launch the ball left or right, at either a high or low angle. High-angled balls travel a short range, while low-angled balls travel farther distances. When used in midair, the spiked ball can stall Piranha Plant's descent. The ball's damage decreases every time it hits an opponent, and deals no damage if it falls to the ground.