As with any punishment, smacking can be abused by parents. I'm sure you'll agree that smacking in the heat of the moment (or in anger) can easily cross the line to become abuse. For a younger child that can understand, they can be warned that a naughty (dangerous) action will result in a smack. The child then makes the choice if they engage in that activity - and they have been pre warned of the consequence. All a smack needs to be is a small tap on the hand, really.
When we were older children, and we knew that we had done something bad, we were sent to our room (maybe to let parents calm down - yes, we were naughty at times!), and then our parent would speak to us about what we did wrong - if we understood why it was bad, etc. Then it was a controlled smack (not a beating, not with a hose/cricket bat, not demoralising - we didn't have to remove clothes, was not done in anger or frustration) and after that (very important) we would sit with mum/dad, talk about it, say sorry, and give hug and kiss. I can honestly say that I never held grudges against them for this method of discipline, as I understood I had misbehaved.
Obviously it is only one way to help your kids learn, and each kid responds differently to different methods. Some you only need to talk it through/explain it to them and they already are in tears/remorseful. I guess with any method, the key is for parents to remain calm and controlled, do it motivated by love to help the child grow, and take controlled action (not out of anger) - which isn't always easy tongue