Master Chen says that separation of yin and yang is rotation in today's terms. He also mentions that stretching is an instance of separation of yin and yang. Today, I came to a realization that when the circle becomes infinitely small, rotation and stretching are really one and the same. The key is that the setup needs 3 points, and one action, which is the rotation, at the centre or the pivoting point or the fixed point. The rotation at the centre is the only thing I can do to ensure that the other 2 points are opposites but of the exact same size. For example, if I just move the hands in opposite directions, since there is no relationship between them (no 3rd point linking the two together), there is no guarantee that their actions are the same in size. At the beginning, I can visualize how the dantian acts the centre and modulates the other 2 points (hand and foot). Hopefully in the future, I can place the centre/fixed point anywhere inside or outside of the body as desired like Master Chen said.
In order to materialize this theory in practice, more training in foundation and yilu will be the way to go.
More discussion occurs at http://practicalmethod.com/lang/en/2011/03/separation-of-yin-and-yang-2.