This exercise is used to widen your lats, but also works your biceps hard. This exercise can help you to do more pull ups, by replicating the movement with less weight.
Begin by putting a long bar attachment onto a lat pull down machine. Next, sit down on the lat pull down machine and place your knees under the pads. Make sure that the pads are set at the correct height; your feet should be flat on the ground when your knees are under the pad. Reach up and grab the bar, with your palms facing forward and a grip wider than your shoulders. This grip will help to isolate your lats. Begin the movement by pulling the bar down towards your chest and driving your elbows back. You should slight lean back, so you can touch the bar to your chest. Once the bar touches your chest, hold it there and try to squeeze your shoulder blades together; this will force you to completely contract your back muscles. After you pause the bar on your chest, return the bar to the starting position; make sure you feel a stretch in your lats at the top position. Repeat this motion until your set is complete.
Your core should remain motionless throughout this movement; there will be a tendency to cheat by swinging your torso. This could cause injury, and will take the focus off your back muscles.
You can use different grips on this movement to work your muscles a bit differently; a closer grip will work your biceps more, and a reverse grip (palms facing you) will work the rear deltoids and biceps.