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.
Originally coined "The Bear" by John Davies, this series of five exercises in one teaches the body to move with great ROM in a fluid, powerful, and fast manner. It's fantastic as a total body warm-up, improving overall power production and motor skill coordination in a short period of time.