In taiji the body (and therefore the mind) contains both full and empty (substantial and insubstantial) all the time. As the classics say, full and empty should be clearly distinguished. One side of the body is full and the other is empty. One side stable and secure allowing the other to be free and expressive. This is cross-energy – the line from weighted (stable) foot to opposite empty hand behaving as a whip. The freer the line is of tension (holding-on) the snappier the energy. Without this principle of single-weightedness taiji contains no lightness – no humour – and becomes what my teacher used to call "sack-of-potatoes taiji."