From the cited study's abstract:
Our findings support the theory that retrieval practice enhances learning by retrieval-specific mechanisms rather than by elaborative study processes. Retrieval practice is an effective tool to promote conceptual learning about science.
Students asked to recall what they had read (a quiz) ended up retaining 50 percent more information after a week than students relying on other methods. The other study methods included repeated studying of material (ie, poring over it) and concept mapping.
Actively recalling information changes the brain's structure. "What we recall becomes more recallable in the future", said Dr. Bjork, a psychologist at UCLA.
One reason students shy away from active recall and quizzing is that it readily reveals gaps in knowledge. It's one thing to read a passage and another to set it aside and provide a list of the major concepts and terms from memory.
You'll initially draw a lot of blanks. It's important to not get frustrated and quit. Rather, revisit the material and then test yourself again.
Cramming by re-reading a sheet of notes can provide a false sense of security.
One of the great things about flashcards is that they provide a way for you to quiz yourself. You have to actively use them, though. Looking at one side and having a vague idea of what the other side says may not be enough. Force yourself to say what the other side says, then flip the card to see if you were right.
Cardboard for iPad, my flashcard review app, has built-in support for quizzing this way. It also includes a memory matching game that can be used to test your knowledge. To get the most out of it, you need to anticipate the other side of the card in all situations.