The D4 Descender features a unique and innovative progressive cam action, which offers control at slow or fast speeds. The progressive cam works by way of one cam operating inside another cam. It is also easy to back-feed the rope through the D4 and therefore it can be used for smooth ascending or for emergency hauling (when used as part of a 2:1 ‘Z-rig’).
The D4 has a rated load of 240kg (529lb), making it a true Work/ Rescue Descenders. It is not necessary to build in additional friction to achieve these loads, although it can be useful to do so, for an even higher degree of control in Belay applications. The minimum user weight is 60kg (132lb).