The simplest instantiation of the Testing effect is elicited when students test themselves on a prompt which they’ll later be tested on (i.e. identically). So: say you memorize a thousand prompts about a topic. What we really care about is not just your ability to answer those specific questions, but how that practice affects your utilizable understanding of the topic. What can you actually do with it? If I ask you an open-ended question involving the knowledge you practiced, can you marshall that knowledge—i.e. Transfer learning.