Inertia. The torque applied by electromagnetic force on the rotor plus that of friction and air resistance equals the torque applied (indirectly through a gear train) by the mainspring. The net torque is zero and the rotor keeps going at the same rate of rotation.
The integrated circuit counts how many times the rotor has spun compared to how many quartz crystal oscillations have happened and adjusts the electromagnetic force somehow. My guess is by increasing/decreasing the power passing through some resistor (creating heat).
The integrated circuit counts how many times the rotor has spun compared to how many quartz crystal oscillations have happened and adjusts the electromagnetic force somehow. My guess is by increasing/decreasing the power passing through some resistor (creating heat).