There are high focus and low focus modes of thinking that serve different purposes in learning something.
Chunking is the encapsulation of several steps or processes into an overarching process. Think of all the steps required to put on a shirt. You need to know how to hold the shirt, have an intuitive understanding of cloth dynamics, pull it in a certain way so that you can put your arms in, etc. But because you do it every day, it has become second nature and all you have to think to complete the task is "put on shirt." It has been chunked.
Chunking requires 3 things: focus, understanding, and practice. Math is largely about chunking underlying concepts so you can learn more complex concepts.
Chunking is a high focus activity, and spaced repetition is best as a low-focus activity that can be used to efficiently memorize jargon and other things that involve rote memorization.
There's a lot more in the book, but those are the pieces that I remember best and that have had the biggest impact for me. I review flash cards every day, and once every few days, I do a high-focus learning activity. Even though I don't put much effort in, it feels like I am learning as much as I did in university.
