maybe it's like the same idea as dry firing a bow. you're letting the engine go up and down the revs fast as f@#k without giving the energy produced anywhere to go so it has a better chance of resonating and breaking things?

I'm guessing if he slowly goes up the revs then backs down it's all good but maybe just wailing on it like that as if he's driving a old carborated motorcycle trying to keep it running is just hard abuse.

If you've ever manually opened the butterfly on a throttle body with a car running and you do it fast like this you can see how hard the engine is torqueing on the motor mounts. it's really trying to offload that energy and when it's not going into the drivetrain it's more apt to resonate within the engine.

Source: these are all my assumptions and guesses, so grain of salt me :-/

 

Makes sense, not that I do anything like this as it always felt like abuse on the car, I was just kinda curious