Just to drive some of you nuts, I will make the comment that I have assigned youngsters of about 11-13 years of age to take over the teaching of the adult class.
This is partly the reason for many Japanese instructors to not say anything when instructing. What they say will have no meaning to the student or will be interpreted incorrectly. If the student simply watches and learns, then the student can interpret the movement at their own level of understanding.
I guess this is a complex way of saying: "Just shut up and practice."
I've been one of those beginners that Rock has asked to teach class (to other beginners). At the time, having a year of Aikido and a judo shodan, I could teach what little Aikido I knew, and help with ukemi. Being a professional rowing coach with a post-grad biomechanics degree didn't hurt, either, but that's another story.
Re: the no talking instruction and practice - I guess the traditional Japanese instruction is being shown by research to be a very effective way to learn movement skills - it's called "intrinsic learning" - people learn more by observing a movement and then trying to repeat it based on what they've observed (sometimes it needs many demonstrations) but with NO COMMENTARY in the demonstration. R. Masters - do a google search on that name with Intrinsic Learning, and you should find lots of academic articles...
These days when I'm coaching, and developing coaching education programmes, I use "try this" and "try that" and "how did that feel", rather than give a great long explanation about how to do things.