This exercise is used to build mass in your back while widening your lats. The wider grip helps to work your lats harder.
Grab a pull up bar with a grip that is wider than shoulder width; the wider the grip the better. Hang from the bar with your arms extended and look up towards the ceiling; this will cause you to lean back slightly and work your back muscles. Begin by pulling your torso up towards the bar until your chin is above the bar. Also, try to touch your chest to the pull up bar. Hold yourself up at the top and squeeze your back muscles. After this begin to slowly lower yourself back to the starting position, and allow yourself to hang at the bottom position. Repeat this movement until you have completed your set.
If you are not strong enough to do pull ups, you can either have a spotter hold your legs or use a pull up assistance machine. Also, if you would like to make this movement more difficult, you can hang weights form a belt of hold a weight in between your legs.
You can vary your grip to work your back muscles differently. This motion can also be replicated on a lat pull down machine; however, pull ups are much more difficult.