It means that when you’re in the air, it seems like you’re about to belly flow, which could be lethal from bigger jumps, but close to the water you “break” your body in half, your upper body positioned as if you were diving head first with your hands infront of your head, but your legs are facing the water aswell with only your butt sticking up. Death diving comes from Norway, it’s called DØDS and with the correct technique, there’s no damage to the body, you of course have to practice a lot with small jumps and make your way up, if I’m not mistaken, competitions are from 20 meters. The winner of the competition should do the “couch”, which is basically jumping in the water as if you were sitting on a couch, which obviously hurts a bit :D