Actually its not a load of bollocks.
I used to use that technique all the time to clear the plugs on C210's to save running up engines over loose gravel. If the prop was cycled a few times after locating a fouled plug, at around 1700 rpm, then nine times out of ten it would fix the situation without having to resort to high power runs. Obviously that is only good when it is a fouled plug not a magneto problem. I heard about the technique at a forum with Richard Collins (from Flying magazine) at Oshkosh a few years ago. He said it increases the pressure inside the cylinder effectively blowing the plug clean. I think that was a very simplified explanation but I could care less - it works.