In reply to timparkin:
Interesting video.
Looks to me like he was a completely inexperienced abseiler, judging by the way he completely let go of the brake rope. But nevertheless the prusik should have stopped him falling.
There are 2 likely possible causes for that. The first is that the prusik on the leg loop may have been too long, and therefore hadn't fully tightened before being stopped from doing so by contact with the belay plate. This almost certainly explains the initial fall, which happened when his leg was particularly high.
But it doesn't explain why, when he dropped his leg again, the prusik then failed to bite - unless the prusik was actually way too long, which didn't look to be the case.
I suspect the failure for it to work once the fall had started was due to a combination of the prusik cord being too fat for the rather skinny single rope he was abseiling on, and the fact that everything was dripping wet, with consequent reduced friction.
If this latter explanation was the whole story (which is possible), then presumably it wouldn't have mattered whether he was on a leg-loop prusik or an extended plate, it still wouldn't have had enough bite to grip. It then comes down to what the novice would have done with his hands in terms of grabbing at ropes above (as in the video) or below (more likely with extended plate) the belay plate.
It's a good reminder that if you are using a leg-loop prusik, to make sure it's very short indeed, but possibly a better lesson to all of us that fat prusiks won't easily work on thin, single, wet ropes.
Post edited at 14:26