The patented extruded hole design, in conjunction with the groove profile of the sheave, provides a more powerful grip than conventional designs - up to 18:1 holding power. Unrivalled low weight is achieved by the use of a Torlon^® ratchet pawl, high grade machined alloy sheave and black Acetal hubs and bearings. On-Off switching knob is fitted to both sides of the swivel blocks (one can be removed if desired), and for foot blocks simply turn the sheave upside down before installation to change the sense of rotation. Unique On-Off switch mechanism can be operated under load. Ratchet blocks can be disassembled for servicing or customisation.