The other day I talked about "The Teaching Window" on my blog. I think it will give people food for thought so here is the link:
The Instinctive Edge
The Instinctive Edge
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Wow, you need some line breaks in that!
Your post seems to make teaching out to mainly be demonstrate and copy style teaching, and it makes some sense that this would have a limited time of application, particularly if you can't scale it appropriately to the audience. In my opinion, good teaching is a lot of work in developing systems for different learning styles, including good communication tools, progressions, reinforcing strategies and cues.
Thus I think that a really great teacher is always getting better and should never reach the end of a "Teaching Window" unless they can't walk and talk anymore. Sure a diminishment in actual athletic ability may hamper one's ease of demonstrating to a predominantly visual learner, but there are plenty of ways around that difficulty.
Those that can not teach because they can not do, are poor teachers, and those that can teach despite never having done are the kinds of people who make Olympic coaches.
But, at the same time, there are levels perhaps of teaching... and sometimes, it's very hard to go backwards.
I've seen this with several very senior, very well respected instructors. Brian mentioned Hatsumi in his blog; it's clear, when you watch videos of his current class, that he is really teaching only for one or two people in the room. The rest will get something out of it -- but not nearly as much as the one or two ready for that high level instruction. Most are going miss a lot of the subleties because they are so refined.