The whole idea is to have the engine at operating temp but not higher. If you run a new engine with an optimal fuel mix it will overheat and either shut off (if you're lucky) or be damaged. The up and down needle valve routine is all about letting friction do it's magic for a few seconds then flooding it with oil and fuel to keep it lubricated and cool enough. Don't run a new engine at a 4 cycling rich mix for long though or you'll ruin the liner's fit by breaking it in too cold. Dub Jett wrote up an alternative method that I use that (IMO) is far more fool proof. You put extra oil in to bring the lubricant up to around 30% and run the engine about 1k rpm off peak and propped to turn the rpms you expect to fly at.