ORIGINAL: ASSOCIATED_DRIVER
Notice he is holding the tires, this puts some load (resistance) on the motor. He also checks the temp with a meter.
The risk on breaking-in by freewheeling (on blocks) is overheating. Since its not moving, there is no wind to cool the motor. And without resistance, the rpm will be high. You'll have to tune it just for break-in, then re-tune it for running.
what are you talking about? an idling motor is almost impossible to over heat, in fact its hard to get the motor up to temps when its idling, the faster the better to 200f anything below puts far to much strain on the motor's parts and in worse cases cause the con rod to snap.
Again he does put some pressure on the tires to make the motor work harder to get temps up, new motors need heat fast.