The passage you are referring to describes the role of the edges curve and how it handles edges…This is a older video. DT used to call it sharpness. But its a very good demonstration and explanation I think about what you are asking… specifically around 9 min give or take but the whole video is certainly worth a watch…Robert made some excellent videos a few years back https://www.youtube.com/watch?v=zzVXK4eAM5E