More correctly, the reference is as to an 'air screw' which if you consider a right hand air screw, you have to screw it through the air like a screw, turning it right at the top or 'clockwise' with a screw driver, in this instance the motor.
Of course a left turning propellor is a left hand air screw, it turns anti-clockwise to move forward.
Hope this helps.
Now, Farid - the price is $650, it is cheap... take it.
Regards,
Nic.