Pai Chang then asked Wu Feng, "With your throat, mouth, and lips shut, how will you speak?"
Wu Feng said, "Teacher, you should shut up."
Pai Chang replied, "Where there's no one, I shade my eyes with my hand and gaze out toward you."
The fundamental practice of Buddhism is transcending the distinctions between subject and object, between self and other. Speaking assumes a subject and an object, a speaker separate from a listener. Pai Chang is asking Wu Feng if he can transcend this duality, if he can express himself with conceiving of a "self."
Wu Feng tries to point out to the teacher that by asking, he himself seems to have fallen into the duality of "self" and "other." But the nimble Pai Chang immediately replies that when he looks toward Wu Feng, he sees no one - not that he didn't see a person there, but that he didn't perceive what he saw as an "other," separate from the "self." Given that, he wasn't "speaking" as a speaker separate from the listener. To Pai Chang, it was all one.
Wu Feng went on to become a great master in his own right, eventually becoming as nimble as his teacher, not sticking to dualities. He once asked a monk, "Where are you coming from?"
When the monk replied that he was coming form the village, Wu Feng asked, "Did you see a ox?"
The monk confirmed that he had, and Wu Feng asked, "Did you see its left horn or did you see its right horn?"
When the monk couldn't answer, Wu Feng spoke for him and said, "I don't see left or right."