In reply to JJL:
I maybe wasn't clear, I understood autoblock to refer solely to a prusik when used as a fail-safe for abseiling - perhaps that's incorrect?
Either way if a prusik is used for abseiling then it replaces your hand in case you let go, the descender takes your weight even though the prusik or your hand still holds the rope. You wouldn't have a hope in hell of holding your entire weight with one hand wrapped around a pair of 8mm ropes, the force needed to hold the rope below the descender is minimal and that's all the prusik is doing, not holding the weight of the climber. If it did more people wouldn't clip it to their leg loop.
Of course you could tie it above the descender and then it would take all your weight but I've never met anyone who does that, the only time I can think you would would be if you were going to have to pass a knot but then that's probably quite a rare occurrence.
Post edited at 10:09