Plain English Meaning
The propeller doesn't just push air straight back — it spirals the air around the airplane like a corkscrew. That spiraling air hits the tail off-center and tries to swing the nose to one side. Pilots counter it with rudder.
Full Definition
The corkscrewing flow of air produced by a propeller as it spins, which wraps around the fuselage and strikes the vertical tail at an angle. In a typical single-engine airplane with a clockwise-rotating propeller (as seen from the cockpit), this rotating airflow strikes the left side of the vertical fin, pushing the tail right and the nose left. The is most pronounced at and , such as during , , and .