If you're struggling to master a complex topic, a new language, or anything else that seems to strain your brain, Michael Nielsen has a suggestion for you: Try flashcards. Nielsen is a scientist and a research fellow at  Y Combinator, and he's written books on such brain-straining topics as quantum computing and neural networks. 

In a recent series of tweets, he explains his process and why he finds flashcards so useful. Silicon Valley insiders have been passing his advice along and asking for details about how he does it. 

Nielsen says he first started memorizing flash cards (using an app called Anki, although there are lots of options) two years ago. Since then, he's memorized more than 9,000 flashcards, reviewing them while doing things like standing in line for coffee or riding in transit. He says he spends a total of about 20 minutes a day reviewing flashcards.

The results are impressive. Nielsen says he's used flashcards to build up his understanding of complex topics such as AlphaGo, reading the same paper multiple times, pulling out bits of learning to memorize on flashcards each time through, until he had absorbed the whole thing. He used a similar method to memorize the contents of a (short) book. He also uses the same technique to remember places he likes in neighborhoods he doesn't visit often. Once he's learned something using flashcards, he never forgets it, he says.

Nielsen has a simple rule: If learning something could save him five minutes in the future, then he'll put whatever it is onto flashcards because it takes less than five minutes total to learn things this way. "The expected lifetime review time is less than five minutes, i.e., it takes < 5 minutes to learn something...forever."

That certainly sounds worthwhile. But just because this technique works for Nielsen, is it likely to work for anyone else? Yes, because using flashcards engages the brain in several powerful ways:

1. Spaced repetition

This is what Nielsen sees as the reason for his success. Research has long shown that we absorb information better when it's repeated, but also when we have time to rest and reflect between study or practice times. I discovered this when I took a Segway tour. For the first half of the tour, I found riding the Segway a little awkward and I was unsure of myself. Then we took a 15-minute break and when I got back on, I found my brain had somehow assimilated what it had learned during the first part of the tour. I was suddenly able to ride the thing with confidence. 

Repetition helps you retain what you've learned, and spacing out that repetition gives your brain time to absorb it. It's a powerful combination that can help you learn and retain almost any kind of information.

2. Active recall

The best flashcards have a question on one side and the answer on the other (or concealed within the app). When you look at the question, you make a mental effort to remember the answer before you turn over the card and look. That effort to search your own memory banks for the issue is called active recall, and it's better for learning than if you were simply reading text or picking answers in a multiple-choice quiz. 

3. Metacognition 

Metacognition is the act of thinking about thinking, and it too contributes to better recall, research shows. According to flashcard app Brainscape, when you use flashcards and check your answers, you're constantly asking yourself how close your answer came to the answer on the flashcard, and whether you really knew the answer or took a lucky guess. All this wondering how you did is a form of metacognition and it will help the things you learned stay deeply embedded in your memory. And it's one more reason you should consider using flashcards for everything you've ever struggled to learn.

Nielsen tweeted that the biggest benefit to using flashcards is that memory is "no longer a haphazard event, to be left to chance." Instead, he can now guarantee he'll remember the things he wants to. "It makes memory a choice," he tweeted.

What will you choose to remember?