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.
Lots of guys can pump out 20. Of course, their range of motion is so short, it looks more like they're convulsing than actually doing legitimate chin-ups. Here's how to change that.