Purpose of Exercise: To widen the upper lats. This exercise allows you to do chins with less than your total body weight, so you can do a lot of extra reps for the upper back. It should not replace chins as the standard exercise for the widening the upper lats.
Using a long bar, grasp it with a wide, overhand grip and sit on the seat with your knees hooked under the support. Pull the bar down until it touches your chest. Making the upper back do the work and not swaying back to involve the lower back. Release, extend the arms again, and fell the lats fully stretch.
Variation: Try doing lat pull downs behind the neck instead of in front.