This is truly a double-duty exercise. It serves two purposes. It builds explosiveness on the concentric (pulling) portion of a chin-up because each rep is re-set at the bottom of the movement.
It also takes you from single-chinner to multiple-chinner. This could be the result of increased grip strength and scapular retraction, which becomes a necessity since there's no need to swing, hump, or kip your way up.
Waterbury talks about his new e-book, Larry David, his gig with Rickson Gracie, and oh yeah, training and stuff. In fact, we're heading off to the gym right now to test drive his little dip, chin, and deadlift training circuit.