That is called the alpha role. In wolf packs, one of the tactics that the alpha male uses (usually as a last resort) is this alpha role. You see it in domestic dogs as well, when a dog/wolf challenges that authority role, the alpha grabs the offending dog by the scruff and forces the opponent on it's back, with the belly exposed. I really don't like the use of this method, but occasionally, it has to be done to prove a point to the dog. It might look like he is using a lot of unnecessary force, but have you ever tried to wrestle a dog that size and pin him to the ground? It takes a lot for force, especially for a GSD. To me, it doesn't look like that form of training seems to be working for that dog, and they should have moved on to a different tactic, but that's just my $0.02.