Teaching is most effective when done by a practitioner of the subject matter. That is why Bret Victor says this:
Real teaching is not about transferring “the material”, as if knowledge were some sort of mass-produced commodity that ships from Amazon. Real teaching is about conveying a way of thinking. How can a teacher convey a way of thinking when he doesn’t genuinely think that way?
The entire blog post is worth a read – and a re-read.