My father smacked me precisely once - I threw a large toy at an antique fireplace during a tantrum. He stepped out the room a moment, collected himself, explained why he was going to smack me and gave me one across the bottom, over my trousers. I was about eight and he never raised a hand to me again - I honestly think he felt ashamed and only did it because that's what his dad would have done.
My mother would hit me a couple of times a week. I was never deliberately naughty because I was terrified of her snarling rages, but any mistakes or delays in meeting her (often impossible) demands could result in an attack. It was utterly capricious - a bad grade at school could mean her pretending to be sympathetic, or repeated slaps in the face. She might 'forgive' something when it happened, but later in the evening suddenly attack me out of nowhere for something completely unrelated that she might have ignored another day. When she stopped hitting me it was because it was unnecessary - if she moved towards me I'd already be flinching and crying and begging. I just needed to see the the rage she was hiding and I'd be terrified.
When the school arranged for a psychological assessment for me, having finally recognised signs of abuse, my mother completely gamed the system. Such a small demure polite woman couldn't possible be abusive, and as I refused to implicate my father that must mean I was deluded or a liar. At least now she was under scrutiny and couldn't get away with physical abuse, though she continued to leverage my trauma to emotionally abuse me. I have to pretend, like the rest of my family, that it was all fixed and everything is cosy now - but I utterly despise her.
I know why she is like this - she was brought up by her physically abusive grandmother and is reenacting what was done to her - but that just reinforces my contempt. She has an opportunity to be a better person, but decided to torture her own children instead.