An old concept was called the Center of Pressure for an airfoil. This was where the airfoil could be "suspended" such that all the forces in front and behind that point were balanced. In other words it didn't try to pivot at all. This concept fell out of favour because at the zero lift angle for any of the airfoils the Cp could be shown to be infinitly to the rear of the airfoil. And physicists don't like dealing with infinite values at all.

The old Cp concept was replaced with a standard using the aerodynamic center that is located at the 25% chord point and then the airfoil is given a pitching moment value or torque value that describes now much it wants to try to diverge when pivoted at that point for various angles of attack.

Now here is the reason you want to pivot it at this point.... on almost all sort of normal symetrical airfoils the pitching moment is very close to 0 over a wide range of angles of attack. That Pm = 0 means you will see the minimum load on the servo and that's a good thing.