http://www.slowtwitch.com/...ving_a_Bike_163.html
The bottom line is that the force in front of the mid-point is greater than the force behind the mid-point thus creating a torque about the hub that resists rotation.
Edited to say that Tom's article is even better.
The bottom line is that the force in front of the mid-point is greater than the force behind the mid-point thus creating a torque about the hub that resists rotation.
Edited to say that Tom's article is even better.