Why explaining reveals gaps
Explaining a topic in your own words — as if teaching a friend who's never seen it — forces your brain to connect the new material to what you already know. The places where the explanation stalls are your gaps, located precisely and for free.
Run the technique
This is often called the Feynman technique: pick a concept, explain it plainly, notice where you reach for the textbook's phrasing because you don't actually own the idea, and go re-learn exactly that part.
Say it out loud
Speaking the explanation out loud works even better than writing it — there's nowhere to hide a vague sentence.