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.
Combos consist of big-money lifts done in rapid succession. Do them right and you'll muscle up quick, along with building speed and power. Oh yeah, you'll also drop some blubber... assuming your heart doesn't explode.