You’ve threaded the rappel ropes through your device, tied the autoblock knot and attached it to a carabiner on your leg loop. You’re now ready to rappel with the autoblock as a safety back-up.
2 Ways to Hold the Knot
Before you rappel, make sure the autoblock is loose on the ropes so that it slides easily. Put your brake hand, the one that keeps you in control, below the autoblock knot and grasping the rappel ropes. Put your guide hand on top of the knot below the rappel device and begin rappelling. Or place your brake hand on the knot and use your guide hand above the device. Either way works fine. Try it both ways and decide which you prefer.
Let the Knot Slide on the Ropes
As you rappel, let the knot slide with your hand keeping it loose. If you want to stop, simply let go of the knot and let it cinch onto the ropes. Make sure you let go of the knot if you need to stop. Novices have died by gripping the knot, which slips on the rope and melts. Let go and let the knot lock.
Avoid Having Your Knot Jam
Make sure that the cord or sling that forms the autoblock knot is not too long. If it is too long, the knot can jam in your rappel device when you stop, which will cause you all kinds of headaches as you work to free it from the device. Avoid problems by making sure the sling is short enough before rappelling. If it’s too long, tie a knot in the end of the sling to shorten it or extend the rappel device from your harness by attaching it to a sling.
Get in the Habit of Using the Autoblock
Get in the habit of always using the autoblock whenever you rappel. When I've climbed in Norway, every climber used an autoblock when they rappelled. In France, the Chamonix guides use the knot when they rappel. But in the US it’s rare that I see climbers use it. Pretty dumb, since it takes 30 seconds to tie, is easy to use, and it can save your life.