When used, Piranha Plant hides back into its pot/pipe. When the button is released, Piranha Plant shoots out and bites viciously - the longer the charge, the farther Piranha Plant reaches. The pot has heavy armor during this attack, and a direction can be inputted to turn Piranha Plant left or right. However, one must be careful when turning, as the pot/pipe cannot change directions after an initial input, and if it lies on its side the attack is stuck at a specific angle, making it easier to dodge. Piranha Plant's hitbox is expanded during the attack, but any knockback taken will start momentum from the pot/pipe. There is also six frames of intangibility on Piranha Plant's mouth during its bite phase. 
This is a reference to the Stalking Piranha Plants that first appeared in New Super Mario Bros. Wii. When they are encountered, the height of their stem changes sporadically, making it hard for the player to jump past.