Discovered by AKA
Get on the side railing next to Jabu's mouth. Get near the edge, but not exactly at the edge, and go into C up. Line up the left side of the B button with the crease where Jabus mouth starts. [add pic here] Now jump straight toward it and jumpslash at the peak or your jump. If you did it right you should hit the loading zone and enter. Sometimes you will get on the top of his mouth and slide but not enter. If this happens, don't give up. Keep jumpslashing when you're about to fall off and eventually you should get in. Sometimes a damage boost from a bomb or chu can help get extra hieght if you can't seem to get high enough.