A lightweight disk turns perpendicular to the air flow.
Meiners’ explanation: “in any other position of the disk in relation to the flow, the lines of air flow are not symmetrical around the edges of the disk. This creates a difference in the air pressures between the front and back surfaces of the disk, thus giving rise to a torque. […] If the surface of the disk were exactly parallel to the flow, equal pressures would be exerted on both surfaces of the disk, and it would again remain at rest. This however, would be an unstable equilibrium,” which is in practice very difficult to achieve.
Location: D4-3