The pinch at the top of the stroke is how max power is made. It is also an interference fit that is very hard on the crankpin and rod. On a new engine the idea is to get the engine up to temp quickly, but still running at a slightly rich scream. Using an undersized prop helps ensure that the load on the engine doesn't contribute to a runaway excess heat condition. The high rpm break in gets the liner expanded sooner, and the piston speed limits the amount of time per stroke that the piston is in the pinch zone. There really is no controversy, any method will work 99% of the time. Most ABC engines I've handled [recently] that weren't sold for competition have very little pinch anymore. It became too much of a headache for the manufacturers' warranty service, because of the old school know it alls who thought they were breaking in a 1960 Fox .40 iron / steel engine, then had to turn in an engine with a failed rod or crank.