I don't believe in smacking at all. My parents did it as conditioning which spilled over to a way of dealing with situations - for example defiance.
As a child at school I was caned, my finger broken, by a grown man wielding a cane but with no repercussions. It was the late 70s.
I think we have to wonder about the way we were brought up. We know corporal punishment has a negative effect; it was normal for us though.
I'm ashamed to say that I reflected my experience sometimes when my DC were little. In a way calculated inflicting of pain (tapping on the back of the hand) is more difficult to justify than the 'flight/fight' reflex.
My mum told me that I was a biter (once) so she bit me back when I was three:
'I never did it again.' she said. How rubbish is that?
I have met two adult 'biters' of approx. thousands of adults. They told me about their quirk so I told them I have a flight/fight reflex. The idiots didn't try it on me.
I'm sure hitting, never mind 'tapping' gently, can become a habit. Don't start it. You have found the point of reflex so work on it.
For the book: I was about 13 when I questioned the practice saying 'go on hit me if it makes you feel better'. I was 17 at the driving test centre when she last lashed out.