Well, actually, it is a very effective method of cleaning carbon from the combustion chambers. When I was still a tune up technician I did it hundreds of times. Sometimes it was done prior to removing heads for a valve job to have less clean up work.
It needs to be done with skill though. If overdone it can damage things. I'm not sure if I would try it on an engine with ported fuel injection since it would be much harder to control than on an engine with a carburetor. (Am I giving away my age here?)
