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.
If you want to correct a few muscle imbalances you may have created before you knew better, or if you want to fortify the integrity of your shoulder joints and maybe even improve your posture, then this might be just the article you need.
The original Bad Santa reveals the best assistance lifts, the best posterior chain movements, and the merits of the box squat as opposed to the standard squat.