Why teaching helps you learn
I've always wondered at how much you can learn at a topic when you make the effort to teach it to others. That fact is the basis of the famous Feynman study technique, and at least one qualitative study suggests it might be explained because it forces your memory to retrieve all the information you've previously learnt.
From my experience, though, I would be more inclined to think that it has to do less with memory than with stepping out of your comfort zone. Not that memory is irrelevant, of course, only it is not the main reason why teaching helps deep learning.
By 'comfort zone' I mean the false certainty of having understood something. Too often, we study a subject and understand it superficially, which triggers a delightful positive emotion that, alas, tricks us into thinking we've got it perfectly. In order to keep searching for answers, we need a feeling of incompleteness, this unease that tells us we aren't there yet. If we don't feel that, we think we're done.
When you teach, you are forced to create coherent narratives from what you know and, more importantly, you have to anticipate the questions your students/pupils/clients will ask, because you don't want to appear as unprepared in front of them. This process of struggling to be as coherent as possible in your arguments (which is impossible if you only have superficial knowledge) and finding all the blind spots you might have actually makes you realise you didn't understand the subject in the first place. Then, after some 30 minutes of imposter syndrome, you get to work and study until you finally think you got it. Which is nice until it is lecture time and someone asks a question that makes your foundations crumble and you have to start the process over again.
Teaching is awesome for learning, and a true expert is someone who has reiteratively gone through this enriching —although sometimes painful— cycle of assessing to what extent they really understand things.
There's a name for our overconfidence in our own knowledge. It's called the illusion of explanatory depth. Basically, you think of yourself as an expert in a given topic until you're forced to explain it. As Leonid Rozenblit and Frank Keil, the authors of this seminal paper put it:
Laypeople rarely have to offer full explanations for most of the phenomena that they think they understand. Unlike many teachers, writers, and other professional “explainers,” laypeople rarely have cause to doubt their naïve intuitions.
We frequently discover that a theory that seems crystal clear and complete in our head suddenly develops gaping holes and inconsistencies when we try to set it down on paper.
Maybe this is one of the reasons theories of change are so important in public policy, and at the same time so dreaded by organisations. They force you to think deeply and acknowledge you were too committed to things you didn't really understood. And that hurts.