This exercise is used to challenge the abs and should only be done with athletes that have a very strong core.
Begin by hanging from a pull up bar with a shoulder width or slightly wider grip. You should be hanging with your arms extended and your legs should also be extended below you. Squeeze your abdominal muscles and raise your legs until they are parallel with the floor. Try to hold this position for at least one second. Next, slowly lower your legs back to the starting position without letting them touch the ground. Repeat this exercise until you are done your set.
Do not allow your body to swing at all, try to keep your torso as still as possible. This exercise takes practice to master, but it is a great ab workout.
Once you have mastered this exercise you may be able to hold a light weight in between your legs. Also, many gyms have an attachment for their pull up bar that will hold your upper arm; this will allow you to perform this movement without having to hold yourself up the entire time.