After a normal wrists catch, swing into the apron and lift your legs up in front of you, pass them through your arms and hold them straight in front of your face (away from the bar) in a whip position. When you reach the backend, push your legs forwards towards the catcher's head (to unfold your body) and catch the flybar.
You can find out more about this trick in my books:
Thanks to Pedro for helping out with this trick.