There is a diaphragm (or piston) with a push rod in the middle, which divides the cavity into two parts, one part is communicated with the atmosphere, and the other part is communicated with the engine intake pipe through a pipeline.
Its principle is that when the engine is working, air is sucked in, resulting in a vacuum on one side of the booster, and the pressure difference on the other side relative to the normal air pressure is used to strengthen the braking thrust. ?