Scenario: You’re descending a route with a team of four, and you have one rope. You get to the rappel spot, which requires two raps on a single rope to get down to easier ground.
Standard practice is to have the first person down set up another rap station (or just wait at an existing station), have the next three climbers rap to the lower station, pull the rope, and then have the whole team rap to the ground. For your team of four, this means eight total raps . . . and a lot of time.
Consider this alternative: Fix one end of the rope at the top. Have the first three people rap to safe ground on a single strand. The last person (a more skilled team member) unties the fixed rope end, pulls up half the rope and threads the anchor as for a normal two-strand rap. Then, this last person raps to the intermediate station and then finally to the ground. This results in a total of five raps rather than eight, speeding up your team’s descent time.
Be sure everyone on the team is comfortable rapping on a single strand, and add some extra friction to the rap if necessary. (An easy way to add extra friction is to clip your rappel device to two biners rather than one.)