I see what it does... sets the heart value to zero. This doesn't work on client though. You see, you can lose all your hearts as a client and relocate yourself and lose all your weaponry but the server will still think you have the same amount of hearts you did before you blew up so you'll probably get killed faster. But yeah, it is pretty stupid.

So really, it's stupid to use it in duels unless they have carrots at the beginning becuase you lose all your weaponry and only relocate yourself, don't get all five hearts back (unless you died with 5 hearts).