This cylinder contains a chamber for the compressed gas to enter and another path for it to leave, there are various different types of ‘action systems’ that pneumatic cylinders and each provides a different kind of force. The simplest version, which is found in most bar stools and chairs, is the single-acting cylinder, where a piston-oriented system forces compressed air through a solenoid valve into the back of the piston.
This highly-compressed air seeks the easiest way to exit, and exerts a large amount of force on the piston face. The surface area of the piston face, or the bore size, directly affects how easily the air will manage to push the piston. The larger the bore size, the more easily the air will move it--until weight itself becomes a significant factor. As the piston is pushed out, the air exits through escape valves that are carefully position further down the cylinder. The piston falls back naturally in place until another burst of compressed air is fired into the cylinder.
If there wasn’t gas lifts in bar stools, it would mean that you have to manually change the height every time, similar to how you would on a bike. The Pneumatic spring in your bar stool will wear out over time, but luckily The cylinders can be replaced quite easily.