The gas cylinder is composed of an outer and inner tube.  The piston separates the inner tube in two chambers.   As long as the valve is open ie. when operating the release lever on the chiar - the ring slot connects both chambers.

As a result, the impact of the chair user's weight causes compression of the gas cylinder.

For the lowest position, a rubber disk provides sufficient spring suspension.  Without a load on the seat, the gas cylinder extends automatically by the inner pressure of the system.

When the valve is closed, both chambers are completely separated and the gas cylinder is blocked, because there is no compensation of the pressure difference betweek the chambers.  In this case the gas volume in chamber provides a spring effect.