This is a variation of the pull up that is much more challenging than a standard pull up.
Grab the pull up bar with a wide grip and your palms facing away from you. Lean your torso back slightly and stick your chest out as far as you can. Now, pull your torso up the left side until your chest nearly touches the bar; this should be done like a standard pull up. Concentrate on squeezing your back muscles and driving your elbows down. Hold this contraction at the top position for a second, then return to the starting position. Now, pull your torso up to the right side until your chest nearly touches the bar. Hold the top contraction for a second and then return to the starting position. Repeat this movement to each side until you have completed your set.
This is a very challenging exercise that should only be performed once you have mastered standard pull ups. Do not swing your body from side to side, control your body.
Weights can be added by attaching them to a dip belt or holding them in between your legs. Take caution when you do this and make sure that you have built up enough strength to handle the weight.