First technical concept set by the author, other technical parts are introduced later in the text. We must draw not only with our eyes but also with our mind. Even to pose oneself, to understand what are the forces that are at play.
The graph curve is forceful: it sums up in one line an idea, with an origin, an event, an end.
Second concept, after that the lines of force, the path of rhythm. If a line or an idea reflected a force, two forces create a rhythm. The sequence of lines is compared to a slalom ski.