Just because you haven't seen it doesn't mean it doesn't happen. The way I was taught to do it is that a dog will naturally squirm and try to break free because they don't like being on their back, in a very comrimising position. Once they stop squirming/trying to break free they have accepted that you are the stronger, alpha, and will not try to challenge you. I did this to my pup when he stepped out of line and after a while they pick up on the fact that the faster they stop moving the faster they are let go. I did it when he was smaller and easier to handle. I can probably still pull one off but he's just too smart now and will stop moving, look at me, and then get let go within seconds.