My Mum claimed not to believe in smacking but did smack me, more than once at a time, on several occasions, usually for nothing. The hypocrisy made it worse. Once, when I was 6, she said that I'd played a note wrong on the piano, and I said I hadn't, and she said I wasn't allowed to contradict her when she said I'd played a note wrong. A few minutes later, she said I'd played a note wrong when I knew I hadn't, and I instinctively said I hadn't, and she chased me up the stairs, threw me on my bed and smacked me several times. She'd engineered the whole situation to assert dominance.
Once, when I was about 14, my brother and my Dad were away, she hit me because I'd used up a roll of toilet paper too fast (?!), so I hit her back and we ended up having a full physical fight, which she won. Of course she did, she was a very physically fit adult and I was a weedy 14 yo the size of a 10 yo who hated all sports. She will never back down from an argument, even if she knows she is wrong.
It was usually when my Dad wasn't around, although he did give me the odd smack- in fairness it was always when I was being a terror and behaviour most kids would have got smacked for in the 90s. At least he owned it and didn't give sermons on why smacking was wrong.
I remember going through most of my childhood being scared of her and for a good portion of it I hated her. It wasn't just smacking, she was highly controlling and came up with cruel and unusual punishments for all sorts of things which weren't even bad behaviour imo.
We were low contact through most of my early adulthood.
We have a fairly good relationship now. She denies any of those events ever happened and claims she was a warm, lovely, gentle mother and I have a wild imagination to remember her as overbearing, cruel and angry.
She still makes underhanded spiteful comments from time to time and I pretend I don't hear them. She no longer affects me with her behaviour, but I still bear the impact of how she was in my childhood.
I would never, ever lay a finger on my DD and will never engage in the authoritarian controlling tactics I endured either. There is never any need for it; good parenting is about prevention and setting your children up for success, not punishing them for failure.