P Factor
 

I've considered the 'bend' in the flight of a soccer ball where the ball is spinning around the vertical axis. In this situation one side of the ball is travelling faster (forwards) whilst the other side is travelling slower (backwards) roughly the same was as props and rotors with p-factor. The faster side experiences more drag (skin friction) had higher pressure and the slower side less drag, and lower pressure as the air is 'helped' backwards by the spinning ball. Therefore the ball moves to the slower side.