While holding an enemy, pressing DOWN while in midair will perform a piledriver. After gaining a bit of height, Peppino will start to fall down, destroying blocks and enemies in its path and killing the held enemy upon landing. Performing a piledriver from high enough can destroy metal blocks. Holding JUMP as you fall into additional enemies will cause you to bounce off of them.
Piledriving enemies (or Gustavo) into bosses will immediately damage them on contact, which can potentially be faster than throwing an enemy into them and following up with a proper hit.