Sometimes just called "eight", this device is most commonly used as a descender, but may also be used as a belay device in the absence of more appropriate equipment, although it does not provide enough holding power for this to be recommended.
It is an aluminium (or occasionally steel) "8" shaped device, but comes in several varieties. Its main advantage is efficient heat dissipation. A square eight, used in rescue applications, is better for rappelling than the traditional 8.
Figure eights allow fast but controlled descent on a rope. They are easy to set up and are effective in dissipating the heat caused by friction but have a tendency to put a twist in the rope. Holding the brake hand off to the side twists the rope, whereas holding the brake hand straight down, parallel to the body, allows a controlled descent without twisting the rope.
An 8 descender can wear a rope quicker than a tube style belay/rappel device because of the many bends it puts into the rope. Many sport climbers also avoid them because of the extra bulk an 8 puts on the rack. However, many ice climbers prefer to use the 8, because it is much easier to thread with stiff or frozen rope.
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