Depress the handle, the pressure in the cylinder should register about 50 bar. Leave it depressed and adjust (if possible) the working pressure (second gauge). Then open solenoid and adjust needle valve.
First gauge should stay on roughly the same number (room temperature depending). Second gauge should too while running.
When solenoid closes, and the handle is still depressed, gas will still escape that's in the tubing. If you then released the handle, the gauges should stay the same as the CO2 is trapped in the reg. that's one way if checking for a leak on the reg. it should stay there till its all opened again.