ACV rinses help to remove some types of build-up from the hair and close the hair cuticle. Think of your hair as being made up of rows of shingles, like you'd see on a roof. These hair shingles, or cuticles, can be in a "open" position (depending on various things), which makes the hair strand feel rough to the touch and look dull. Being that hair has a naturally low (slightly acidic) pH, things like vinegar rinses can help close the hair cuticle and lock in moisture and protein. Vinegar rinses can also help reduce tangles.
The pic on the left below shows hair with cuticles open; the pic on the right shows hair with the cuticles closed.
As for how often to do this, I think once a week would be sufficient. You don't want too much of a good thing, though - over-acidifying the hair can lead to damage.
I hope that helps!