Actuators are the devices that move the control
surfaces on an aircraft; they are installed into the overall control circuit in
one of two ways - either connected in parallel or in series. When connected in
parallel they allow the control column to follow the movement of the control
surface (ie if you bank to starboard the control column will reflect that). On
the other hand, most of the time the control surfaces are also moving very
slightly to counter minor deviations in flight path, and under these conditions
it would be undesirable for the control column to shuffle around continuously.
A series connected actuator is therefore made to be very stiff at the control
column end in order to negate any column movement. Large modern aircraft have
both parallel and series actuators fitted.