We are talking about the wing twisting moment. On a symmetrical
airfoil at zero lift, zero alpha, the pressure patterns top and bottom are the
same, so there is no residual tendency for the wing to twist - zero moment.
On a cambered airfoil you have to go nose down alpha to get zero lift. In this
attitude the zero lift is the sum of some positive lift at the back of the
airfoil and some negative lift at the front. This pressure pattern produces a
residual nose down twisting moment - even though there is no overall lift.