[Question] How to identify "pivotal chapters" in HtRaB?

https://www.lesswrong.com/posts/isL6XMQswKjc7Ri2W/how-to-identify-pivotal-chapters-in-htrab

In the Systematic Skimming part of Adler and Van Doren’s How to Read a Book, they say you should identify the pivotal chapters. I couldn’t find a good definition of this in the book. Does anyone know what they mean?