According to Wikipedia the testing effect means ‘that long-term memory is increased when some of the learning period is devoted to retrieving the to-be-remembered information through testing with proper feedback’.
If you did not understand that, neither did I the first time. Basically it comes down to this: “You more easily remember information when you engage with it, than when you just consume it”
Even shorter: “if you want to learn something you have to work with it”. If you just re-read information, you only recognize what you’ve seen before. Your brain likes that because it wants to be energy efficient. But without effort, knowledge won’t stick. Engagement is the key.
UX is, of course, all about engagement. Good design creates a dialogue with the user; it suggests the right questions and provides proper feedback at the right time so the user can learn. That’s the real meaning of engaging design.