After you throw an enemy into another enemy, jump on the cluster of defeated foes and quickly press B as if you wanted to pick them up. Instead of picking them up, your character will pick up nothing! The game acts as though you are really holding something though, which means you can't go in doors unless you press B to throw your "nothing" away. Tip: Try hitting the second enemy on a bounce to give yourself more time to jump onto the dying foes.