The autoblock knot, an easy-to-tie friction knot or hitch that is tied around a climbing rope with a thin length of cord, is used as a safety back-up knot when you’re rappelling. The knot is the best back-up simply because it does two jobs exceedingly well: It locks under load and, unlike all other friction knots, it releases while still under load.
An Essential Safety Knot for Rappelling
The knot, tied below the rappel device, slides down the rope as you rappel. If you stop, the knot tightens and cinches onto the rappel rope. When it cinches, the autoblock knot stops you from rappelling if you let go of the rappel ropes. The autoblock knot is an essential climbing safety knot—one that every climber should know how to tie and use. In Europe it's called a French Prusik knot.
Use an Autoblock When Rappelling
Rappelling is one of the most dangerous aspects of climbing since you’re relying solely on your equipment, your anchors, and your climbing smarts. It’s important to take every possible safety measure to minimize the risks of rappelling. You double check your rappel device. You double check the anchors your rope is threaded through. And you use an autoblock knot on the rope as a safety backup.
The Autoblock Keeps You in Control
The autoblock knot allows you to safely stop and hang to clear rope snags; toss the rope farther down the cliff; free twists and knots from the rope; keeps you from losing control, especially on free rappels; and stops you if you get hit by a falling rock. The autoblock also allows you to rappel slowly and stay in control, especially on free or overhanging rappels where you are not able to touch the rock.