Originally Posted by
hokiefyd
If you observe the pedal itself, it actually spins BACKWARDS (or rearwards) as you pedal forward. Counterclockwise on the drive side and clockwise on the non-drive side. Thus, if your pedal bearing freezes stuck, the pedal will actually loosen itself as you pedal forward (a good thing, rather than breaking your ankle). To my understanding, this is why the pedal threads are as they are.
That's true, but the motion and pressure cycles do actually serve to exert a tightening force on the pedal, not a loosening force. I'm afraid I'm not going to be able to explain it particularly well....but there's a Sheldon Brown article on it somewhere.