Swing the bar and hook your knees over it once it returns to the board as you hang from the platform supports. Once you are ready, grab the bar with both hands and you will swing away. Depending on the rig layout where you fly (and particularly the distance from the platform to the bar) you may also be able to leave the platform without grabbing the bar, holding on only with your knees. Be careful of your head if the platform is very close!
You can find out more about this trick in my books:
Thanks to Tim for providing an image for this page and to Ted for the video.