Can't remember the name of the book, but it was very entertaining. The one I quoted before I altered this post may not be it.The wikipedia page has some links though.
At one stage, they thought that the Germans had not seen their fake tanks, and a German tank column was approaching. So they faked a couple of flashes, as if the sun had accidentally reflected off their armor. The Germans saw and turned around, which was just as well, as the war may have gone quite differently. It was a crucial moment in a particular battle.