Basically, if you get knocked out and die, but then you somehow enter a Left-Right or Up-Down Level Splitter before touching any ground, you'll "come back to life" with zero health points. However, this makes you "invincible" until you either replenish your health OR you respawn.
EDIT: Moved from Level Designer Bug Reports to Chapter 2 Bug Reports. I don't want this bug to be overlooked.