Klemheist Knot or French Machard knot
The Klemhiest is a form of slide and grip knot. Whereby the knot can easily be moved along an object (usually a rope). However when tension is put on the end, the Klemheist will lock in place. One disadvantage of the Klemheist is that it will only lock in one direction.
The Klemhiest is often used by rock climbers and Arborists to assist in the ascending a rope. The idea is that as a rope is climbed the knot is advanced up the rope. Then when the climber stops, the Klemheist locks in place and allows the climber to rest with the knot taking the weight.