When abseiling from an anchor during rock climbing, I like to use a backup prusik as a potential lifesaver in case I screw up.
However, I have heard different advice on where to put it, both for where to attach it to myself and where to attach it to the rope:
One option is to attach a Prusik above the descender but below your guide hand then clipped to your harness. Your guide hand pushes the Prusik down as you descend.The disadvantages of this method are:
Better therefore, a Prusik can be attached below the descender and clipped to your harness leg loop. Care must be taken to ensure the Prusik can never enter the descender, potentially causing either to fail. You brake hand goes above the Prusik and pushes it down as you descend. The advantages are:
Freedom of the Hills has a chapter on rappelling that includes a detailed discussion of this topic.
Tie it "uphill" from the belay device or "downhill"?
Downhill.
If the Prusik is downhill from the ATC, then the Prusik only ever has to supply enough force to brake you on the brake strand. This is much, much less than your body weight, which is what you'd need if you had it above the ATC.
An even more important reason why it's a terrible idea to put the Prusik above the ATC is that it has a really terrible mode of failure that can kill you. It goes like this. You have your left hand above the ATC, tending the Prusik, while your right hand is on the brake strand below. Something happens (rockfall, getting banged around, ...) and your right hand goes off the brake strand. In this panic situation, you grip with your left hand as tight as you can to try to save yourself. This has the effect of preventing the correct tension in the Prusik, so the Prusik doesn't function. The rope strips the flesh off the palm of your left hand, and you fall to your death.
Make sure to add an extension between your belay loop and your belay device, so that the Prusik can't possibly get pulled up into the ATC.
Clip it to the belay loop or the leg loop or to a loop of my personal anchor system (PAS)?
For many harnesses, as long as you have the Prusik downhill from the ATC, it doesn't matter. The Prusik only has to supply braking force, and a leg loop, for example, is plenty strong enough for this. However, for some harnesses the force on the biner can pull the leg loop open. Therefore it may be safer to make a habit of clipping it in to somewhere more secure.
This answer is purely complementary and does not attempt to answer the question directly.
Another option for securing a rappel is the fireman's belay. This works on single pitch climbs when someone is at the base of the cliff, or on multi-pitch routs when someone is already at the next, lower anchor.
The belayer takes a hold of both strands of the rope, while keeping in mind to give the abseiling person enough slack to smoothly feed rope through the rappel device. In case of an emergency, the belayer pulls the rope strands tight, which causes the belay device to lock up.
I use this method whenever possible, since it is much quicker than attaching and detaching a prussic. It also eliminates the difficulty of resuming a rappel after the prussic has bitten down on the rope. In case of an emergency, the person below could safely lower the person rappelling, which is not possible with a self-belayed-abseil.
These points make the firemen's belay the preferred method when helping a novice to rappel. Someone new to ropes, harnesses, and rappel devices can concentrate on the more immediate aspects (avoiding overhangs and jerky movements,) while the belayer takes care of the unexpected, and can take an active roll in lowering, if necessary.
Some people swear by never even using a prussic, and instead lowering the first person off of the anchor, and then using the fireman's belay for the second. This methodology should only be used when lowering off of one's own gear, since the weight of the climber and the friction of the rope quickly abrades fixed rappel rings.
Benefits are: It becomes impossible to rappel off of the end of the ropes when the backup knots were forgotten (just watch out for not lowering the climber off of the end of the rope,) and if it turns out that the rope is not long enough for a double-roped rappel, the climber can climb back up on top-rope, instead of having to awkwardly climb on self-belay. I have also heard that this method can be quicker than using prussics, although I don't have any data to substantiate this claim.
Drawbacks are: When rappeling the traditional way, it is possible to extend the second's belay device with a sling, which makes it possible for both climbers to set up for rappel and check each other's work. Since this is not possible when lowering a climber first, that method should be used for experts only, since a novice should never set up an abseil without having his/her work checked (no-one really should.)
