The testing effect (retrieval practice effect): testing yourself on material produces better long-term retention than re-studying the same material for the same amount of time.
This seems obvious once stated, but conflicts with students' intuitions. Re-reading feels productive because the material feels familiar. Testing feels uncomfortable because you're aware of what you don't know.
The mechanism: retrieval strengthens the memory trace in ways that passive exposure doesn't. The effort of retrieval is the learning.
Practical implication: flashcards beat highlighting. Practice tests beat re-reading notes. The uncomfortable thing is the right thing.
Related: desirable-difficulties, ↗ @alice / how-i-take-notes