I am new to this training thing with horses, but I have tried different training methods and find that some trainers feel hitting is OK. I don't agree, I've trained dogs for years and even had a rabbit and guinea pig who walked on leashes and have always trained with firm discipline and kindness, but you do have to be in charge. Smacking them in a way another horse does, as in a nip on the butt or a full smack to the chest as corrective as so many of you have said sounds totally reasonable and works.
But I agree with those of you who are against this man who kicks them in the rump to motivate them, I find a Tiny bite from a dressage whip works great or even a dressage bat, the noise will send my girl flying with a tucked butt and it is only noise.
I have been watching videos (free) on statelinetack.com made by Chris Irwin, I got a line on from another forum, and he makes sense, teaches how to think and interact with your horse. I tried Clinton Anderson and the tap tap tap pissed my arab off but this method I use now by C. Irwin is working for her, she's such an angry soul (a 4 yr old rescue, couldn't let her be dog food). Hope this helps, from one novice to another. I also try Gawani Pony Boy, but only the ground, no round pen. Sorry so long!