But minor differences in airspeed also exerted unbalanced torques on the ball and caused it to spin. Similar flow imperfections between the two plates created differences in viscous drag and exerted torques on the two plates. That's why they began to spin around slightly. A drag force is a force that opposes an object's motion through a fluid. Like sliding friction, drag always pushes the object in the direction opposite its motion though the fluid.
Air resistance is really a drag force.
You feel drag pushing you backward when you ride a bicycle fast. If you were to fall downward, you would feel a drag force upward, in the direction opposite your motion through the air. Incidentally, the object pushes back on the fluid with drag force, too, and this force on the fluid pushes the fluid in the direction opposite its motion past the object. This force tends to stop moving fluids and to turn their kinetic energies into thermal energy. Suppose that you fall out of a plane about 30 seconds after your parachute pack fell out.