The Stomach Under is usually performed on a bar to bar rig. This variation is easier to catch with a catcher as you make a half twist after leaving the bar which allows you to catch wrists rather than a stick. Swing out and make a half turn at the frontend. Over the platform, bring your legs over the bar like a pullover and hang with the bar on your waist, legs straight. Just before the peak, reach for the catcher and make a half twist as you straighten your body.
You can find out more about this trick in my books: