It is just about regular old spanking, isn't it?
I agree. A swat on the butt can reinforce the message without doing any physical harm. With young children, a verbal message sometimes benefits from a little reinforcement. A smack on the face, however, is never called for.
It is much the same with training a pet. You don't want to make them afraid of your hand. There needs to be a fundamental relationship of trust. Touch is an important part of that.
With pets -- and probably also with children, although to a lesser degree -- the timing of the swat is also important.
My dog sometimes runs off. When he comes home, my gut instinct is to punish him. But that is wrong. He will think he is being punished for returning home.
You have to deliver the swat right when they are right in the process of doing the act you wish to discourage. It also does not need to be painful. Loud is good, however. I got my dog when he was about one year old. He had apparently been mistreated by his original owners. Probably by children. He would trust me, but he wouldn't let anyone else get near him.
It took two years for him to get over all that. Mostly over it, anyway -- he'll probably never be fully over it.