This is a double-duty exercise. It serves two purposes – it builds explosiveness on the concentric (pulling) portion of a chin-up, and it can help you perfect a proper pull-up.
Each rep is re-set at the bottom of the movement.
You'll benefit from this exercise if you're a beginner or an advanced lifter, and it can take 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.