Learning a body of knowledge can be tough and for some us overwhelmingly daunting. So we look for quick wins. Skimming, frenzied cramming, and even plagiarising were options most of us opted for at least in some of those pressurised moments at school. Inevitably all those options led to knowledge leaking away within a matter of days. There are times when that didn't matter (my Chemistry GCSE); the quick win served a purpose.
Most of what we learn in life is pretty essential to how we operate as humans though. The knowledge we acquire shapes our lives in a myriad of ways. Without knowledge we couldn't have built relationships, got around the place or fed ourselves. These are such essential activities that the knowledge behind them is taken for granted, but it was tough and at times overwhelmingly daunting to learn effectively. It is knowledge that has stuck though, precisely because it was difficult to learn, thus making difficulty desirable.
Every time you test yourself you "not only measure knowledge but change it" (Dr Roediger, Washington University, St Louis). By change Roediger means we encode the knowledge differently, which can be subtle, or sometimes dramatic!
Say you learned that the Vietnam War ended in 1975, and a day later someone asked you when it ended and you correctly answered 1975. By bringing the knowledge out of your memory you enrich the information and encode the knowledge back into your neural pathways slightly differently because of the change in context around being asked. A week later you're asked again by a different person, and again answer 1975 correctly (thanks to being tested a week before). This time you relate the date to an image you saw during the week of helicopters evacuating civilians from the US embassy roof in Saigon. By relating pieces of new knowledge and making connections with prior knowledge you have changed your knowledge of when the Vietnam War ended - it still ended in 1975, but it ended then in a particularly chaotic manner. Your knowledge was measured (do you still know it is 1975) but it has also changed (it was 1975 and it was chaotic for the last Americans in Saigon).
If over an hour period you had repeatedly read over your notes on when the Vietnam War ended, then sat a test and got the answer right you would have crammed the knowledge. If not tested again, after a few weeks the knowledge would almost certainly have disappeared.
So if you want to learn large chunks of knowledge or knowledge that is complex, you must review that knowledge regularly through testing. Better still, every time you review your knowledge through testing you will make sure it is difficult. You want to have got close to forgetting the knowledge so it is harder to retrieve. This is desirable because the knowledge will be reinforced in your memory, even if you couldn't remember it. Checking the answers will help encode the knowledge.
Desirable difficulty does NOT mean you spend more time learning. Actually it translates into less time spent learning, but learning is spaced over a longer period of time and only occurs when it will be difficult. That is what makes learning stick, and that is what Feed your Elephant is all about.