Its my mistake. I thought self flagellation meant something like "self-harm with a purpose or religious intent", instead it specifically means whipping oneself as part of a religious ritual however I think thats basically the same thing.

Here is the text I was talking about but this is really the densest place he talks about it, the rest is scattered, a few sentences here and there throughout the rest of the lectures and I cant assemble that.

https://www.jordanbpeterson.com/transcripts/biblical-series-XI/

It’s also a permanent marker. I’ve read a fair bit about practices like tattooing and body scarification. Those are very, very common practices. They’re human universals, actually. No matter where you go around the world, you see a couple of things. First of all, almost without exception, people wear clothing. Sometimes it’s relatively minimal clothing, and often it’s more decorative than protective, but it’s almost inevitably clothing. The other thing that you see is that people do scarify and tattoo themselves. They do that, in some manner, to catalyze their identity. They’re trying to transform themselves from a generic person, in some sense, to a person that’s been designed by their own hand. It’s something like that. It’s a marker of developing identity, and some of it seems to be catalyzed with pain.

Modern people who tattoo—and I’m not saying that I’m in favour of tattooing, because actually I’m not, but that’s my own particular bias, and if you have a tattoo, that’s fine with me. I’m also not saying that there’s anything wrong with it. But one of the things you do see is that people with a tattoo do report a couple of things: the pain is actually necessary, and that the pain is actually something that seems to establish something like a memory. Well, it’s a memory because of the pain, because you bloody well remember things that hurt. But it’s also a memory because it’s actually etched on you, right? It’s not something that you can just abandon and forget. And so a circumcision is exactly the same thing. You don’t forget it, because it’s part of you. It makes a good token for a covenant. That seems to be the rationale, here, speaking from a psychological perspective. It’s to indicate, as well, that the damn thing that’s happening is serious. I also think that was the case with the earlier sacrifices of animals.

Modern people don’t do this. You don’t know how serious you would take a vow if you sacrificed a goat in your backyard. It’s actually a very dramatic thing to do. You think about it as primitive, and perhaps it is primitive and archaic, and no doubt it is. But it’s also to take the life of something, and to spill its blood. That’s no joke; that’s something you remember, especially if you haven’t done it before. We actually don’t know what we would need to do in order to take something seriously. We all do things like make New Year’s resolutions about how we’re going to be better people, and we don’t do it. The reason for that, at least in part, is because we don’t know how to make the sacrifice sufficiently bloody, let’s say, so that we remember that it’s necessary. We don’t take it with seriousness. We don’t think, ‘I should quit smoking, because I’m going to die.’ We don’t think through what that means: coughing your lungs out for three months in a hotel bed while your entire family is repulsed, horrified, and sorrow-stricken at the fact that this has happened far too early. We won’t make that real enough to make it serious. Without that seriousness, we won’t do it. There’s something to be said for rituals of seriousness. I think this is one of them.