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.
How many Mondays have you been to the gym, anxiously awaiting your chest workout, only to find that you have to wait 45 minutes just to get on a bench? It seems that every guy in the gym trains his chest on Monday; it's a foregone conclusion.