Girl clicks horse. Horse learns how to be brave. Alexandra Kurland recently wrote a nice piece about spooking and how to help the horse to control his body when a spook overtakes him. This also applies to asking my horse to relax when he hears a click. He must be taught how to do it.
Survival runs deeply in the horse and fear is it's natural counterpart.
Fear pumps him up and gets his jets firing. Once it's started, it can be hard to stop. We've probably all experienced the power of fear in a horse. In my mind, the click calms because it contains a promise that is never broken. A promise that only good things are to come.