follow the little air line to the switch. when the switch is turned off, or when pressure maxes out and motor shuts off, a little lever presses on the little air release valve at the end of that little air line. this relieves the air pressure in the lines, so that the compressor can restart easy on the next startup.
one way to see that valve working. get compressor running then disconnect cord. then turn switch to off. you should hear an air hiss sound briefly as pressure is released.