It's not about wacking a horse, or bossing the horse around. It's about respect. What people don't understand is that in a herd, they are "disciplined" by other herd members much, MUCH worse than anything CA does, or you or I could do. Just yesterday, one of my horses (950lb QH) was "picking on another"horse. Well, he got the crap kicked out of him by a 1250lb draft horse, with shoes!!
He had one slight slice on him from the shoes and that's it.
There is NO WAY I could inflict that much discipline to any of my horses. I'm just not strong enough. Because of that, they MUST respect me, or I'm in big trouble. Horses are big and can be dangerous. I recommend seeing his ground training videos. He can explain it much better than I can. But he does sum it up like this. As gentle as possible, as tough as necessary. (paraphrasing here)