The old water torture technique. It's for clearing carbon deposits as far as I know, and you let it take water in in bursts through a vacuum pipe I believe, might be completely wrong, but I've not tried it personally. I was present when they tried it on Steve's car I think.

Pete

I've used STP in very high mileage cars or those with very bad piston slap. In those cases it has made an improvement so might be worth a go. I wouldn't consider using it in an engine which is running 'tight' as it will clog the oil.