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Undeniably, the course is helpful for native teachers. What I am curious to know is why
only for native teachers.
IMO, regardless of anything, while speaking everyone is doing something with his mouth, lips, teeth or whatever he has to pronounce something meaningful. Why not use these activities to polish one's sounds?
On the other hand, if these activities can help learners start speaking English more naturally, why do you think non-native teachers cannot benefit from them as well?
Incidentally, A. Underhill is addressing a group of non-native teachers at the seminar and actually shows how a Spanish-speaking teacher gets her /ae/ right at last. Or, are we talking about different books?