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.
Want to build great abs? Forget sit-ups. Want great biceps? Forget all those curls. Want a great chest? Forget the bench press. According to Chad, isolation movements are a waste of time. He's either nuts or a great visionary.