Swing out and get into a Stomach Under position, with your hips resting on the bar. As you swing towards the platform, let go of the bar and reach up towards it. When you get to the peak of the swing over the platform, grab the uprights and immediately straighten your body so you come off the bar. You will have to hold on tight as your body swings down, so you can place your feet on the board. This trick is easier on rigs where the platform is quite close to the fly bar.
You can find out more about this trick in my books: