Wing Chun is a close-range art. Pin Sun maybe emphasizes this a bit more than others. We pivot just enough to get the job done. When you are close, the pivot is often small. So it comes from the Kua/hips and can be very fast and helps deliver "short power". When a larger pivot is needed from a bit further out, the feet adjust for the increase in amplitude. But even though the motion initiates from the hips, it is powered by the legs and the feet pressing into the ground. There is no torque on the knees when done properly because the feet move naturally to compensate. All footwork motions, including the pivot, are centered at and controlled by the Kua.