It is well to moor your bark with two anchors.
Publilius Syrus (a.k.a. Publius) Maxim 119
Chapter 5-Chapter 7 covered how to tune SQL with the diagramming method but did not discuss why the method leads to well-tuned SQL. With a lot of faith and good memory, you could likely get by without knowing why the method works. However, even with unbounded, blind faith in the method, you will likely have to explain your SQL changes occasionally. Furthermore, the method is complex enough that an understanding of why it works will help you remember its details better than blind memorization ever could.