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 nutrition and training info is top notch and so are the writers. I really enjoy Eric Cressey, Mike Robertson, Dan John, Alwyn Cosgrove, Charles Staley, Chad Waterbury, and Christian Thibaudeau. I've read much of what they've written.