After a normal wrists catch, swing into the apron and just after the peak, 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) making a half twist at the same time before catching the fly bar.
You can find out more about this trick in my books:
Thanks to James for providing a video for this page.