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.
Most of you just have an ab, as in one non-distinct blobby area between your pecs and your naughty parts. Here's how to make your singular ab into plural abs.