Research shows that people who (wrote a summary) remember 50 percent more material over the long term than people who follow (repeatedly read). This is because of one of deep practice’s most fundamental rules: Learning is reaching. Passively reading a book— a relatively effortless process, letting the words wash over you like a warm bath— doesn’t put you in the sweet spot. Less reaching equals less learning.
On the other hand, closing the book and writing a summary forces you to figure out the key points (one set of reaches), process and organize those ideas so they make sense (more reaches), and write them on the page (still more reaches, along with repetition). The equation is always the same: More reaching equals more learning.
I think that explaining the material to someone else (either by writing it down or by speaking to an imaginary student) it also forces you to be sure that you understand the material.