the first time a motor (read this as new block or motor straight from the factory) is run... you should take lots of time to ease it in because the heat cycling is changing things inside of it.. this cycling is necessary for the engine to reach proper sizes and clearances internally for smooth operation..

With a rebuilt motor, everything (or most everything) that is in the motor (or rather the block) has already been heat cycled, and after a very short time it is fairly evident if the rings have been seated or not.. at this point using this "full throttle" method is acceptible and possibly better.