This passage alludes to the results of a pattern of rigorous discipline that informs many “performing arts” (which would include the tea ceremony and calligraphy as well as theater) in Japan, as well as East-Asian martial arts.  Nō is exemplary in this respect, since its forms of diction, gestures, gaits, and dance movements are all highly stylized and extremely unnatural. The idea is that one practices for years a “form” (kata) that goes counter to the movements of the body and thus requires tremendous discipline—to the point of a breakthrough to a “higher naturalness” that is exhibited when the form has been consummately incorporated. This kind of spontaneity gives the impression, as in the case of Grace, of something “supernatural.”