Well I haven't read the book but I'm sure he's talking about using the mirror while squatting during actual work sets. That makes sense because your head won't be in the correct position. However, while learning the form you *have* to be able to get visual feedback from what you're doing or you'll never be able to learn it properly. Also this learning process is with very light weight or no weight at all. You see what you're doing is correct and then you learn what that feels like. Then once you've learned the action and how it feels to do it correctly, you start adding weight without a mirror.

Make sense?

IronWorker