From an Uprise position, roll forward to hang under the bar and continue your swing.
This is easiest to achieve at the frontend of the swing, but is also possible at the backend. For the backend version, the flyer can optionally let go of the bar and then catch it again (similar to a Front Hip Circle).
You can find out more about this trick in my books:
Thanks to Pneumatic Arts (@pneumaticarts) for providing a video for this page.