I'd be interested to see what the universal measure of "A Quick Slap" is - location, pressure, speed, what warrants a repetition, and whether they get harder in relation to the discipliner's frustration.
From the NSPCC Website:
Hitting your child if they have done something you think is naughty simply doesn't work. Your child will copy the way you act, so if you hit them, they learn it is ok to hit other people. They may turn into bullies at school, and even hit back at you next time. If you catch yourself about to lash out, count to ten. It may calm the whole situation down.
Seeing red; exploding with anger; wanting to hit your child because you've tried everything else. If you reach this level of anger, it has turned into rage. Rage is always destructive, hurtful, and if you experience it, you will need to get some help.
Click for some extra information.