Smacking, like raising your voice, is a tool which CAN be damaging for children when it's misused. That doesn't mean it's always damaging.

I firmly believe that when used correctly, for SOME kids, it can be very effective. Kids need to be shown boundaries and taught behaviour regulation. For many there's a developmental stage where they just don't have the cognitive ability to reason through an explanation of why their behaviour is wrong, or understand the link between their behaviour and consequences. The instant, unpleasant, physical feedback of a smack can be the simplest and most effective way for them to associate that behaviour with a negative response, and learn not to do it again. It works because it communicates on a reflexive level, not a reasoning level.

I don't smack my kids. I don't ever want them to fear me like I feared my father

My kids have never hit each other or verbally abused me or their siblings. Ive also never smacked them, there has just never been a time when a smack has come about i cant imagine any of them being so bad that id need to hit them. It seems weird.

Use violence to stop violence. Great idea.

It’s illegal where we are so not something to debate

Nope it doesn’t work and does do harm !

Smacking can be effective in that it gives a negative outcome to a behaviour. So essentially - instead of your child only receiving positive reinforcement for their actions ie. annoying a younger sister and getting a reaction out of them they are also getting a negative response (the smack). Although like most behaviours - children learn through observation. So if a parent is releasing their frustration out on the child by lashing out and smacking them ... they are essentially teaching the child that it is okay to hit people when they are angry. And yes, children can tell quite easily when you are simply disciplining them or when you are frustrated and angry. Now the difference in the two smacks is in the application. So smacking can be a terrible form of disciple. Or it can simply provide a negative feedback to quickly and effectively change unwanted behaviours. A lot of people use the fact that they were smacked and turned out okay as a reason to smack their own children. If done properly is perfectly acceptable in my opinion. However, children can be trying and difficult and a parent trying to justify lashing out in anger because they got smacked too is not ideal. This is not a good way to disciple (but we are all human). Although it does get results - it also negatively reinforces to children that they can also lash out when they are angry using violence.

I was smacked, strapped, belted etc! Turned out happy and successful. Love my parents!
I smack my kids :), irrespective of what others say or think :)

It works with some kids. But it must not be done in anger.
You can explain to some kids why not to hit a sibling.
But if it doesn’t work there needs to be an escalation of punishment until something does work.
Taking away something the offender likes is often effective.
But the bully must be stopped. And if it takes a taste of their own medicine, that needs to be applied.

The question should be why he is abusive to family members not about smacking

You could try? A once off might have some success?

There's no point asking these woman i've seen before. They'll come down hard on you.