A passive controlled system has no servo motors itself but reacts to the changing environment. A typical example is the ball on the beam problem. The ball itself has no motor inside. If the position of the ball should be changed, the motor of the beam has to be put into another direction. One way in doing so is a PID controller.
The more demanding control task after a ball balancing one, is a passive dynamics walker. This is an underactuated legged robot. Another example for an indirect controlled system is a juggling robotarm who has to throw a ball into the air and catch it.