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.
Combos consist of big-money lifts done in rapid succession. Do them right and you'll muscle up quick, along with building speed and power. Oh yeah, you'll also drop some blubber... assuming your heart doesn't explode.