In the very end, go through the door leading to the final boss fight, go to the top of the shack thing, and keep jumping between that and the area around the other ebtrance, eventually you will hit solid AIR! Keep pressing O over and ver until you reach a certain hight. Then you can run around and mess with the physics of the game!

P.S. Be patien, sometimes it takes work.
Videos at youtube.com search "Kingdom Hearts Glitch"