Frame shifting is a type of feint; it is a way of distorting your opponent's perception. First, you create a frame. Raise your arms as you would if a robber said "Raise your hands, this is stick-up!" Move both arms inward until they are just outside of a shoulder width apart, with hands open and fingers extended. The arms now act as your guard and also create a "visual frame" that your opponent must look though to see you, and that you must look through to see the opponent. Mentally, this frame acts as does the frame of a painting; you see the painting but tend to ignore the frame.
Once the frame is established, you may manipulate it to deceive your opponent. As your opponent punches, instead of blocking the punch, quickly jerk both sides of the frame laterally a few inches. The tendency is for the opponent to reference the trajectory of the punch to the sides of the frame instead of at the picture (your face, the target) inside the frame. When the frame is shifted, the tendency is for the opponent to keep the punch referenced between the sides of the frame, even though your face is no longer within the frame. This cause the punch to miss its target without you ever touching the opponent's arm.