This fantastic device from the boys at ART is both an ascender and descender in the one unit! The ART Spider Jack works just like a climbers friction hitch by pinching the line when weight is applied.??The Spider Jack requires only a few feet of rope weight before it starts pulling through on its own. The Spider Jack always stays close to the harness and its low profile design makes it a dream to use.??This high performance and responsive device is designed for experienced climbers.