This exercise is done on a pull-down machine, and really targets the biggest muscles in the back, the lattissimus dorsi. Sit down on the bench, and keep a nice, straight back and neck.
Try to keep your shoulders down as you pull the bar down in front of you bending your elbows and using your back. Slowly straighten your arms back up to straight without lifting off the seat.
Make sure you control the speed of movement, do not let the weight pull you back upwards, keep a nice steady motion, and keep your breathing steady throughout.
Repeat x 20.