It looks like he moved maps around and then recompiled it. http://forum.gbadev.org/viewtopic.php?t=9318 There's a second video in that thread too. Apparantly he aslo had it working where enemies would give you a star every time you beat them.
Heh, those things are always fun. Remember the debug level that could be unlocked with an Action Replay in Zelda The Wind Waker?