I think I am getting it, and it's only because re-watching the start of the vid he makes clear that this is an abstract method of drawing. Meaning that we don't have a technical/measurable reason for making some of the lines (like the ones of the face), but we make the lines because they imply a shape or feature in a way that makes sense. Like that the front of the head should be a straight line to imply it's flat because that's the front. Or how you make a triangle to imply a nose because that line gives us enough information to see a nose without having to draw more. I guess a rhythm is like, the visible pattern of how body parts flow into each other.