While midair, pressing DOWN (or the body slam button if enabled) will prompt Peppino to crash down into the ground, breaking marble blocks and killing enemies on contact. If performed from high enough, Peppino can also break a metal block upon landing.
When the move is first performed, Peppino will rapidly accelerate down, with seemingly no maximum speed limit. Body slamming into a slope also allows for the move to be linked into a dash. These two facts combined make the body slam effective both as a combat tool and a movement option. This is even more true if you master the powerbomb for quick access to this move.