Think of a guided rappel as a zipline and a rappel combined. Guided rappels are useful for avoiding obstacles like water at the bottom of a rappel. Instead of rappelling straight down, you can rappel out as well. Secure the "zipline" rope at the top and bottom. It should be tight enough so that when it is weighted, the rappeller misses the obstacle. The rappeler attaches a carabiner on a cowstail to the "zipline" strand and his rappel device to the other strand. The zipline strand controls the path that the rappeler travels while the rappel strand is used to control the speed.
Can be used to avoid obstacles like water.
Requires an anchor at the bottom. Time consuming.