Start in a position with one leg hooked on the bar (a Gazelle), or with the bar between your legs like an almost-Split position. At the backend, hook the front leg onto the bar and simultaneously drive your head and shoulders between the trapeze cables, making a full front roll around the bar. You'll end up in a Gazelle position.
You can find out more about this trick in my books:
Thanks to Rachel and Brenna for providing a video for this page.