If someone can't explain something in their own words, then they don't _really_ understand it. The process of taking time to think through a topic and check one's understanding, even if only for oneself and the rubber duck, will reveal mistakes or points of confusion.

Which gets to the core of the issue nicely, I want to go on to HN and talk to people who know things or have thought about things to the degree that they don't need a cheat sheet off to the side to discuss them.