Strange how polarising this topic always is, particularly since I'm someone who actually swapped sides.
From I was a teenager babysitting my younger brother's friend and his siblings until ds was about 18 months old (when I was mid-thirties) I was seriously anti-smacking. I thought it was abhorrent and couldn't understand how anyone could deliberately hit and hurt any child, let alone their own. And then, well, I changed my mind.
DS started stamping his feet on the buggy wheels so that it stopped dead. Damned annoying on the path, but extremely dangerous if he was to do it when we were crossing a busy road. I tried reasoning with him. I tried taking away his toys. I tried refusing him treats. As we were out and about I couldn't really think of anything else to try until, out of other options and in sheer desperation, I slapped his leg.
And it worked.
I can still remember going home and telling DH that I'd finally found something that worked. And then having to tell him what it was - he too was totally anti-smacking. Unfortunately for both of us it turned out that for a number of things a slapped leg was the only thing which would work for ds, but, other than the pushchair wheels situation (which he did stop altogether after being slapped a few times), he was always given a warning and the opportunity to do what he was supposed to before being punished. We would count to three, but usually he would do what was required after only getting to 1. Of course, gradually the frequency of the slaps reduced until we couldn't remember how long ago a previous incident had been, as he got older and his reasoning skills developed it was just needed less often. In fact, by the time he got to about five we considered him too old to smack or slap except for very serious offences.
DD was different. The slapped leg at toddler stage didn't work for her, in fact it just made her worse. After trying it a couple of times we ditched it as a strategy and worked to find an alternative. However, it turned out that a smacked bum worked for her as she got a bit older, say four, and now at five she's growing out of needing it.
I now think that smacking/slapping has its place as a parenting tool but only if it works for a child. Otherwise it's totally pointless. However, I still find it abhorrent to hit a child in anger.
However, I really do wish the anti-smackers would cease lumping a judicious smack or slapped hand/leg in with abuse as such lack of discrimination is, in my opinion, an insult to those children who have been abused.