I find the highlighted statements in conflict with each other.
One doesn't need to hit a dog to train it regardless of its age. It can work as a deterrence to certain behaviors, but it also teaches other things. Specifically, violence as a training tool teaches the recipient that violence is a tool to be used to get people to behave as we want them to.
I'm no pacifist. There are certainly times when violence is right course of action, which is typically when more or worse violence can be averted.
I haven't needed to strike my kids, not even the euphemistic slap on the diapered butt. As a parent or guardian of an infant/toddler, we have an obligation to keep them from harm. If that means picking them up or otherwise physically moving them, a tactic we wouldn't use on an adult, then so be it. Once they get older, physical restraint becomes a much more extreme measure. I wouldn't do anything physical that I wouldn't be comfortable with another adult doing in the same situation to my own kid.
Fear of physical punishment at best teaches someone to avoid doing something for fear of physical punishment. It doesn't teach them right from wrong. It doesn't teach empathy. A parent may also attempt to teach those lessons alongside of the physical punishment, but it's not inherent to the aversion technique.
More importantly, it's typically in conflict to the lesson being taught. Take a kid who is speaking disrespectfully to a parent. A slap in the face doesn't teach the kid anything about respect. It doesn't teach them how to address people in the proper tone. It tells the kid that parents can treat them like chattel. It teaches them that if they have power over someone, physical violence is a proper response to verbal aggression.
If the kid doesn't mouth off again, the parent assumes a lesson was learned. It wasn't. The kid is keeping his mouth shut because he doesn't want to get hit, not because he respects you. There's a big difference. If you haven't made the kid too fearful, just watch how that kid shows respect to other people.
Like most people on FB, I have a number who put out the typical memes about how spanking teaches respect and so forth. What's interesting is that a number of them, especially one very vocal one, are like little FB dictators when it comes to their walls. People who disagree with them, even in the most polite terms, get blocked followed by some public shaming.
Is that respect? No. They are behaving as they have been taught, which is "this is my house, and what I say goes or <smack> you're gonna get it."
I wonder how many of the verbally abusive trolls (the ones using vile language and hateful comments like how someone should be raped) were the ones raised with spanking and other physical violence. I would bet most are, because when faced with little chance for consequences, they behave horribly. I find it hard to imagine those whose parents used other techniques to teach them respect and empathy are posting rape threats.
On the other side of this coin the "slap in the face" technique doesn't teach kids how to stand up to themselves. I want and expect my kids to show me some disrespect from time to time, especially as they get older. It's only natural to rebel and to disagree with authority. My job is to teach them how to do it properly, which includes when to do it and the manner in which it is done.
If a child grows up fearful of showing any disrespect or otherwise challenging their parents, it will make it harder for them in many aspects of their lives. A couple of good smacks to the face as they are coming into their own can be enough to turn them into people who avoid conflict, like asking for a raise or telling a boss (better still, a boyfriend) "no" respectfully but forcefully.
