This exercise is designed to strengthen your abdominals. You can hang from a chin up bar or use special straps (shown in the picture). This exercise can be done several ways. The first way is bringing your knees to your chest and lowering them back down. This is the easist way to do them and recommened for beginners. The more advanced version of this is keeping your legs straight throughout the entire movement. For those wishing for a good challenge, try bringing your feet to the top of the chin up bar. Whatever way you do hanging leg raises, make sure that you do not swing and use momentum to perform reps. If you are not strong enough to do this have someone place their hands on your lower back. This will prevent this from happening.