I have a specific question and a generalisation of that question.

Specifically, I have recently considered obtaining and working my way through some maths teacher training materials because I want to be better at explaining mathematical concepts (or any concepts, really) to others. I don't know whether this will actually be a productive use of my time. So, a question to educators: are there general theories and principles of this aspect of education (tuition, explaining stuff, etc.) that I could pick up through reading a book, and experience immediate gains from?

More generally, are there any useful heuristics for determining what subjects do or don't have this characteristic of "core principles with immediate gains"? A few hours of self-defence training raise you considerably above zero hours of self-defence training, and reading How to Win Friends and Influence People gives the reader a lot of immediate practical tips that they can start using. Meanwhile, a lot of academic subjects require a considerably greater investment of time and effort before you can actually do anything with them.

I do have a certain level of skepticism as far as this characteristic is concerned. I'm pretty sure someone who's read a decent popular introduction to economics is equipped with a lot of useful principles, but they're probably also equipped with a lot of oversimplified ideas and a great deal of overconfidence in their understanding of the subject.