The Buttstomp is a physical manuever useable by all three characters. It is performed by pressing down in midair. When used, the character will spin around for a second, then plummet straight down at high speeds. Certain blocks will be destroyed, as will the first enemy the character contacts. Buttstomping will also reveal Invisible Gemstomp events.
A buttstomp does negligible damage to bosses (with the exception of the Devil Devan Boss, which is bugged) and two hearts of damage in multiplayer.
While buttstomping, very little horizontal movement is possible, and it does not end until the character contacts something. Being damaged will knock the character out of a stomp, however.
Eat your lima beans, Johnny.