The main purpose of this exercise is to strengthen the muscles of the upper back while thickening the back. The lats, and biceps are also used in this movement. The lower back is also engaged to stabilize the body in the bent over position.
Begin by gripping a barbell with a double overhand grip, this means palms facing your body. Next, bend at your waist until your back is nearly parallel to the floor, but make sure to keep your back straight. Keep your head up, because this will help to keep your back straight. Your arms should be hanging in front of you now. To start the movement, pull your elbows back while keeping your torso still. Bring the barbell to your body, just above the belly button. Make sure to keep your elbows close to your torso, and do not allow them to flare.
Be sure to keep your back straight and still during this movement. This will ensure that you are working the muscles of your back properly, while preventing injuries to your lower back. If you have a history of lower back problems, you need to be very careful with this movement or find a variation to work the same muscles.