Roll backwards from sitting on bar
1 Back Somersault
Swing out from the platform, bring your legs up at the front end, pass them under the bar and hold them straight in front of your face in a whip position. At the backend, push your straight legs over the top of the bar and follow with your body. Let go of the bar and grab the trapeze cables. Pull yourself quickly into a sitting position. Just before the peak at the front end, drive your feet up towards the sky, lean backwards with your hips in line with your body and let go of the cables. Rotate backwards around the bar until the catcher is in front of you. Beginners may find it easier to hook their knees on the bar in the first position before pulling themselves up to sitting. Equally, if the sitting position cannot be achieved in the first swing (it's easy to get stuck with the bar at your knees) the timing can be changed so the catch occurs on the third swing.
You can find out more about this trick in following books: