Prusik / Klemheist knot

A prusik, Klemheist knot, or autoblock knot may be used as safety back-up, commonly referred to as an 'autoblock,' and is used as a back-up in the case of the abseiler losing control of the abseil. One of these friction hitches is wrapped around the rope below the rappel device, using a short loop of smaller diameter cord or webbing, often called a 'prusik loop'. The prusik loop is then attached to the belay loop or leg loop of the harness. [Wikipedia]