Are you asking about a Bourdon Gauge attached to a piston type deadweight tester?
The pressure within the system is regulated by the weight applied to the piston. The system can be either pneumatic (a gas or air) or, most commonly, hydraulic (a liquid, usually water or an oil), or both.
As a load is applied to the piston, in this case by adding or removing weights, the pressure within the system required to keep the piston within a defined range increases or decreases.