For the sceptics, the following is an extract from the Lycoming Flyer Key Reprints, page 66 under the heading of Spark Plug Fouling;
“Prior to engine shut-down the engine speed should be maintained between 1000 and 1200 RPM until the operating temperatures have stabilized. At this time the engine speed should be increased to approximately 1800 RPM for 15 to 20 seconds, then reduced to 1000 to 1200 RPM and shutdown immediately using the mixture control."
This is an engine manufacturers recommendation, which hasn’t been adopted by aircraft manufacturers.
Interstingly, the above procedure is identical to the published shutdown procedure for a number of radial powered Yak aeroplanes.