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.