Straddle whip at frontend, just before catch
1 Back Somersault
Swing out and back. Make a small set at the backend (smaller than a layout), break early (just before the poles) and bring your legs up (wide apart) into the straddle whip position, no sooner have you made this position, but let go of the bar, and unfold the body to catch. When you are intially attempting this trick in lines, you will want to start with the lines between your legs or they will wrap around your legs as you make the rotation with your legs open.
You can find out more about this trick in following books: