Top-down cables are for use with loose luff, asymmetrical gennakers. The tack of the sail is allowed to "float" on a swivel above the main furling drum which allows the torque from the furling unit to be passed along the full length of the cable to the head of the sail where the furl starts and works its way down the luff. Due to the depth of downwind sails, there is a large sail area to furl and therefore top-down torsional cables work at high speed and the torque is transferred over the full length of the cable. It is for this reason that top-down torsional cables are designed to carry much higher torsional loads than bottom-up cables and as a result are generally more expensive. Bottom-up cables should never be assumed to be capable of working as a top-down cable and using a cable with inadequate torsional strength can lead to torsional failure which can result in significant sail damage.