EASA Module 4 · Servomechanisms
A servo drives an output to match a commanded position. The controller compares demand with feedback, and the error drives the motor. Raise the gain to respond faster — but too much causes overshoot and oscillation.
Feedback subtracts the actual output from the demand to make the error signal; the controller amplifies it to drive the output the right way. Low gain is slow and lags; high gain is fast but overshoots and rings before settling. This is how flight-control actuators and autopilots hold a position.