Strike them on their shoulder with their hand or whip? For my horse, it does nothing.
She did it with her hand. I'm no expert, but she is and could probably explain it better. She said it had more to do with that section of the body, than the force. She is even the head organizer for the Winter Classics held down here every year. I've seen her do some amazing things with horses, and she is infamous for taking a horse who is written off in one sport, finding what he excels with, and turning him into a winner at it.