Leg loop verses main belay loop
You'll find recommendations for placing an autoblock on either the leg loop or the main belay loop. Some concerns about either placement are:
- If you're placing an autoblock on your leg loop AND you're using a "quick release" buckle, the carabiner that's holding the autoblock can catch on the buckle and release the tension on that leg loop. It probably wouldn't make you fall out of your harness, but would be uncomfortable, and you'd hang at an odd angle.
- The leg loops of a harness aren't strength-tested independently of the harness as a unit. Also, if the autoblock did manage to catch the entire weight of the climber, it would place a different stress on the leg loop than a person normally weighting the harness would. If you had a harness with stitched leg loops, like the Black Diamond Chaos, the weight of the autoblock would be pulling the stitches apart (this is not the direction of force you have when you weight a harness normally).
- I no longer put my autoblock on my leg loop for the reasons I mention above. I originally did, when I had a different harness, but my 2 current harnesses are the new Misty Mountain Cadallac, and the Black Diamond Chaos, one has quick release buckes, and the other has stiched leg loops.
- If you're placing an autoblock on your belay loop, you have to make sure and extend your belay device when rappeling (using a sling or something like the Metolius PAS). This issue is that if the loops of the autoblock are too close to the belay device, they can get caught in it, and jam the rappel system (probably making you unable to decend the rope, but possibly forcing the rope to "fail open"). If you got stuck in this fashion, it would be pretty difficult to free yourself, esp. if you were in a free-hanging rappel.
Sorry for not giving a single recommendation, as in many situations in climbing, there are multiple options, and its important to know what the tradeoffs are.
As per usual, you want to make sure your carabiner is loaded along its spine. That is, longways. So, one end of the carabiner will be running through the leg loop or belay loop, and the other end will have both strands of the autoblock coord.
What I do
I place my autoblock on my main belay loop, then extend my belay device with a PAS. The rope runs down between my legs, and I can use either hand to control my descent. My autoblock is made out of coord that I measured and cut specifically to be too short to catch up in my extended belay device.