I'll give you an example. When I was three years old I was habitually acting up in church. There was a park next to the church building and mom would take me for a walk in the park so I could run my excess energy off. But after a while, she sensed that I could do better and I was just acting up to get a walk in the park. And since she loved and knew her child, she was right. She warned me that I was a big enough boy that I couldn't do that anymore. But I didn't believe her and I went wild to get my trip to the park. She took me out the door as always, but instead of a walk in the park she took me into the car and gave me very robust and unpleasant spanking. Now that made a believer out of me! I've never, ever disrupted church services since then.
And in an epilogue to the story, a few months later, when I was still 3 years old, I served as the ring bearer at my oldest brother's wedding. Being the wild soul that I am, I disrupted the rehearsal. My mother took me out of the building, promised me that if behaved correctly in the ceremony I would get a toy and promised me that if I disrupted the wedding I would get a whipping. She had already made a believer out of me with the park incident. I was the perfect ring bearer and folks wondered how such a little fellow could have done so well. But it wasn't magic, my mom knew her son and made a believer out of him through corporal punishment. Done correctly, a kid will need it less and less as they get older and will grow up learning that antisocial behavior can have dire consequences.
You hit the key with your story... There is a VERY small window of time in which true corporal punishment works, and if it is TRUE corporal punishment and not abuse it works well to kind of get the child’s attention that there IS a line there. I probably wouldn’t have used the same method for that situation as your Mom, but understand I’m not judging her, and if I put myself in her shoes maybe I would have!
However, there IS another line - and that is the one a parent crosses when the physical lashing out occurs repeatedly, through anger, and accompanied by venom, shaming, ridicule, etc... PARTICULARLY when a child reaches the age that they can truly understand what’s going on. I have to say that any truly physical punishment of an adult to a child beyond that age is DEFINITELY not going to bring about the desired result - even if it APPEARS to have done so (for the time being at least...). Ditto for a child younger than that who even if they can speak are still not going to receive the desired message, and will only learn bitterness and fear.