Elon uses the “Richard Feynman” technique from what I have read about his approach, mixed with “first principles.”
… which is, basically, in simple terms: don’t try to remember, but try to understand; when you understand, you will remember automatically.
Sounds simple? But yet, so many people don’t do it like that. They try to cram loads of info and facts into their brains, especially students, with the result of forgetting a lot of it.
So how does Elon do it? He is quoted on Reddit:
One bit of advice: it is important to view knowledge as sort of a semantic tree — make sure you understand the fundamental principles, i.e. the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to.
So what Elon basically does is, he looks at the most fundamental principle of any subject matter, instead of separating the subject matter into smaller pieces.