What do you learn while instructing?
You learn how much you absorbed from people who told you (your instructors) and whom you at that point believed, but which you cannot really explain.
You learn how to explain things which you took for granted.
You thereby learn how things really work, in so far as you can understand them.
You learn what things you do not really understand, and you learn how to cope with your lack of understanding.
You learn how to get along with people whom you would never otherwise choose to be your friends, via a common interest in flying.
PBL