Smacking kids doesn't teach them discipline or right from wrong, it teaches them to fear you. How you can form a healthy relationship with a child that is scared of you is beyond me.
If a child understands that they've done something wrong, they don't need to be hit and if they don't understand why what they've done is wrong, then they won't understand why they are being hit.
I don't know how (most) adults can understand that it's wrong to hit eachother, but when it's your own defenceless child, suddenly it's ok.
It's absolutely disgusting and I'm so glad that I live in a country where it's been criminalised.