The Klemheist knot, like a Prusik knot, is a friction knot or hitch which allows a climber to ascend or descend a fixed climbing rope, particularly in an emergency situation.
The knot is tied with a thin cord, usually 5mm or 6mm in diameter, onto the rope. The climber then attaches himself to the knot by clipping the cord onto his harness. Progress up a fixed rope is made by sliding the knot up the rope and then weighting or loading it with the climber’s weight. When loaded the Klemheist knot cinches onto the climbing rope, allowing the climber to progress upward. When the knot is unloaded, it loosens, allowing the climber to slide it up the rope.
The Klemheist knot is often used instead of a Prusik knot because it can be tied with a nylon sling and does not tighten on the rope as much as a Prusik knot, which allows the climber to move the knot more easily. The Klemheist knot, unlike the Prusik knot, will also slide down the rope so a climber can descend a fixed rope.
