We were very surprised that my (reluctant first time dog-owner) parents were told to physically pin their dog down as a way of asserting dominance. It’s weird to see that this might be “a thing”.
It just seems like it is likely to make the dog see that as a way of trying to assert dominance, whereas in almost all other cases positive reinforcement (and negative attention from undesired attention) seems to be perfectly adequate.
Perhaps it’s only supposed to be something for the worst-case untrained dogs and has spread out from there?
The pinning is something dogs sometimes do. We have fostered dog moms with puppies and sometimes the mom would put an unruly puppy on its back and grab it by the throat. It seems to be last resort when the usual communication like growling doesn’t work.
The man is simply harmful, tho [1]. His methods promote aggressive approach where you take control by threatening your dog with your body language.
[1]: https://www.youtube.com/watch?v=ppLnTlF8s0k (huge number of dislikes, but talk to anybody who works with dogs professionally)