From swinging facing the platform. Force out over the platform and hold a set. Break as you pass the bottom of the swing, then sweep your legs forwards again.Let go and rotate backwards for a single somersault onto your back.
You can find out more about this trick in my books:
Thanks to Ryan for providing this video.
IMAGE CREDIT: Thanks to Emerald City Trapeze Arts. Flyer is Sarah Piepho. Thanks to David Treadwell for the photo.