Clean dirty pistons while still in engine?

Put it all back together and run a cup of water through the carburetor with it about 2000 RPM.
I thought I was the only person who knew that trick. My High School Auto Shop Teacher, Mr. Farris, taught us that; only he used water in a Windex bottle. He said the water turned into steam and would clean everything as clean as a whistle. I have used it many times.