Locking Off a Figure Eight Rappelling Device
One advantage of a Figure of Eight rappel device is that it can be locked off on the rope, a useful trick for some circumstances. It makes it so that the device can be used as an ascender. A few caveats—it only works with flat faced Figure Eights like the Petzl and SMC, and it only works with some ropes. Please, please, please, practice this at home before trying it in the field. When locked off, the Eight is somewhat iffy, so only use it that way for a few moments.
[A]. The Figure Eight is used in Figure Eight mode.
[B]. To lock off, bring the brake hand strand over the top, and snug it in, between the rappel strand and the body of the Figure Eight.
You may need to pass the strand over your head to get it in the correct position. It should also be noted that by bringing the strand up over your head, you are decreasing the friction through the Figure 8.
So. Learn and understand this in a safe environment. Wear a helmet. Be safe. Learn more tools, and learn which tools to use when. Instruction in this and other techniques is available from these fine local Guide Services.