The Klemheist Knot, also called a Machard or a French Prusik is a friction knot for ascending a rope or escaping a belay.
When tied properly, the Klemheist Knot grips the rope when weighted but releases and slides along the rope when the load is off. It releases and slides with more ease than the Prusik knot but grips with less tenacity than the Prusik. It works well for tying off a climber's rope to allow escape from the belay.
To Tie a Klemheist Knot:
Wrap a loop of cord or a sling of webbing four or five times around the rope and toward the load, keeping the wraps neatly laid against the rope.
Pass the lower end of the loop through the upper end and clip a carabiner into the lower loop. You can add more wraps if the knot slips.