Desirable Difficulty in learning: Harder retrieval leads to better learning, given retrieval is successful. Test yourself before we think we are ready. Recall without a hint.
Many studies have shown that retrieval practice, such as self-Q&A and flashcards, is more effective for learning than review practice. However, we still tend to prefer review because we can't accurately gauge our own understanding. While review feels smoother and easier, retrieval practice is challenging and exhausting. Therefore, the easier one makes us feel like we understand it better.
Psychologist R. A. Bjork adds another concept of "desirable difficulty" to the effect of retrieval: "More difficult retrieval leads to better learning, provided the act of retrieval is itself successful." Retrieval tests without hints are better for retention than ones with hints, and tests that require recall are better than recognition tests (tests to recognize the answer rather than think of one).
The difficulty isn't an obstacle to making retrieval work. It's the reason it works. Therefore, testing ourselves before we think we're ready can be more efficient for learning. Aim for the harder tests.