Some lifters have trouble hitting their backs, even with back exercises. Often, the biceps take over. The unilateral lat pulldown allows you to really focus on the back one side at a time.
Do not fully extend the arm at the top of the movement.
The Ohio Hellraiser talks Blast & Cruise, Harley Davidsons, the pros of bodybuilding training(!), and what conditioning drills rate high on the awesomeness scale. (Hint: it ain't the treadmill).