I haven done a back flip yet, but the most obvious thing to me is .... your head, Take a look at where you are looking. When you get in the air it looks like you are looking at the nose of the ski. Your body will follow your head. On your barrel rolls your head is cranked in front of your roll which is what I am getting at. I bet if you look up above you as you leave the water it will get you around further. I seems ,when you start to leave the water, you just stop your rotation because you stop leading your body with your head by looking at the nose of the ski. See if you can find a video on youtube of a Japan rider doing back flips in slow motion. YOu can see his head looking up which in turn arches his back, which in turn throws his hips forward. Remember, your body will follow your head. Good luck. Points for trying.