Question - What is motion balance principle?
Answer -
A controller that moves its parts to generate an output signal. Moving towards the nozzle is the increase in the baffle. The backpressure of the nozzle is going to increase.
The bellows will extend this change in the backpressure acting on the balancing bellows. As a consequence, the nozzle is pushed upwards. The nozzle moves until movement is almost equal to the movement of the input baffle.