Inversão de orações com expressões como ''not only''

Hey, guys, can anyone explain to me what happen in those sentences, in which the teacher says:
Not only after the prices get lower will people buy it.
And the studant says: People will only buy it after prices get lower.

My teacher said it is a formal ways to speak, but it is mostly common in writings. But why is there a "no" in the first sentence?