ACV smooths out the hair cuticles so they are lying flat like fish scales. Hard water and shampoos make the cuticles stand up and if you can picture what that's like, then you can see how cuticles on different strands can catch on cuticles on other strands and lead to tangles. Also when cuticles are raised, they are split ends waiting to happen because they snag on something (another hair) and get ripped out. Or when you comb, they comb rips them out. So smoothing out your strands - which happens only if your hair has an acidic pH, hence the reason we use the acidic vinegar - reduces the chance of your hair developing weakness that lead to breakage.
I do my ACV rinse at the very end of my wash, because I want my hair cuticles to be closed as the last step of my wash. I mix ACV (about a 1/4 cup) in a basin of water (I have a filter, but when I didn't it was just regular tap water) into which I've added 20 drops of lavender EO, 20 drops of rosemary EO . (When I have them, I usually add 10 drops of geranium EO and a few drops of liquid sage.) I then dunk my head in the solution and massage my scalp while just letting my hair get a good soak in that. The fact that my head is hanging down and I'm giving it a good massage I believe is great for circulation and hence delivery of nutrients to follicles. Also this massage ensures that no conditioner residue that could cause itches is left on my scalp. My head feels oh so clean when I'm done, and my hair smells so clean. This is usually the highlight of my wash day: the ACV rinse.
