I have been working on a Corvair for a couple of years and have spent a lot of time on the Corvair forum. This is a standard recommendation for cleaning carbon out of an engine. The idea is that it creates steam in a hot engine that loosens the carbon that then goes out the exhaust. The Corvair runs at about 350 degrees so it is pretty effective in making steam.
I have seen other people recommend this proceedure on "standard" engines as well. I figure give it a try it couldn't hurt. I did this with a tank type sprayer when I first got the truck and it seemed to smooth up the idle right after I did it.
Keep the revs up and spray in the water with a garden sprayer until you have put in about a quart. Take it out for a good hard drive (the fun part) and your done. Someday when I need to pull heads I will try this just before I pull them. Some people recommend putting in a little alchohol.
I'm not sure this is a good idea on computer controled cars. It will probably mess up some sensor.