It's because the more you work on Mu, the more it will expand in importance. If you keep working on it, eventually it will crowd out all judgments from the mind. When your mind becomes empty of all judgments for a single moment, you'll realize your true nature. You'll achieve a spiritual breakthrough. You'll awaken. Instead of trying to live well, from then on your only task will be continuing to live better and better (by, in koan training, continuing to work on additional koans and letting the resulting insight permeate your life).
