I've seen the Whiffle bat used on a VERY nippy mare who was spiraling into full on biting during even careful and consciensious girthing (any medical reasons had been ruled out). The riders were smaller kids at the time, so this was pretty intolerable. One whack, and the nipping ended instantly, with the horse none the worse for the wear. Like QHChik said, more surprise and dislike of the noise than pain. A hand smacking probly hurts more.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown