I asked Edd about this, and essentially you are putting the reg in the divers face so that's all they see, and simultaneously controlling their upper arm with your other hand. The idea is that what they really want is the regulator, but if they are in fight mode and continue to go for the one in your mouth, you can turn them away from you. You don't want to do anything aggressive because they are already in survival mode. If you know about ocean rescue of a panicked swimmer, it's something similar, you go underwater and turn them around if they try to climb on your head.