The Figure 8 Contingency Anchor is a rigging option that is very versatile. To rig the Figure 8 Contingency Anchor:
Feed the rope through the anchor point until both ends reach the ground.
Connect the figure 8 to both strands as if you were setting up to rappel double strand.
Use a canyon quickdraw to connect the Figure 8 to the anchor. The quickdraw should connect to the webbing and not to the quick link (otherwise it may pinch the rope and prevent the rope from sliding through the quick link during the lower. Note that the picture shows a carabiner connecting to the quick link - this is not recommended.
The last man down removes the quickdraw that connects the Figure 8 to the anchor webbing, attaches the figure 8 to his harness, and rappels down double strand using the figure 8.
Note that if you use the Figure 8 Contingency Method, you can rappel on either strand (i.e. it isolates strands) and both strands have contingency. This speeds up large groups because the second person can be hooking up while the first person is still rappelling. It also gives each person the option to rappel single or double strand.
The Figure 8 Contingency Anchor is very straight forward to convert to a lower. To convert the Figure 8 Contingency Anchor to a lower (if the rappeller gets stuck on rappel):
Remove the quickdraw from the Figure 8.
Un-rig the brake strand by taking the rope off the neck, and out the big hole. Be very careful not to remove both strands (which should be very difficult, but you never know).
Reattach the quickdraw.
Feed rope through the Figure 8 until the stuck rappeller reaches the bottom.
CAUTION: Because the Figure 8 Contingency Anchor is relying on friction through the Figure 8, which is not locked off, there is the potential for the rope to creep through the Figure 8, or even slide uncontrollably through the Figure 8. Small ropes, heavy rappellers, or not-smooth rappellers, may all cause this method to fail. Please test your combination of gear in a safe environment before taking this method into the field. Additionally, it is crucial to have a belayer (either at the top or the bottom) hold the non-rappel strand in case it starts to slip through the Figure 8.