Combined rotation lower leg and foot.
With a closed chain, when the foot is in contact with the floor, there is always a combined motion of the lower leg and the foot. When the foot is making a supination, the lower leg makes an external rotation and when the foot is making a pronation, the lower leg is making an internal rotation. This is good visible at the video, especially the supination/external rotation combination. When you put your fingers at the lateral edge of the tibia, it is also possible to feel the lower leg rotation.