I was similarly skeptical when I heard of this procedure.
Actually, it's good advice. The lead scavenging agents in 100ll do not work at the combustion temperatures at idle. Leaning on the ground will reduce the quantity of lead built up, but will not remove the lead build up. The 20 second run at 1800 RPM gives a high enough temperature to activate the scavenging agents and remove lead build up. Basically you're doing a lead cleaning procedure at the end of your flight.