The device can operate 2 gas cylinders at the same time. One cylinder is the operating cylinder, being used until the remaining pressure in the operating cylinder sinks below a defined pressure. Now the reserve cylinder is used. You have to close the empty cylinder by switching the knob to the reserve cylinder which is now the operating cylinder. You can change the empty cylinder and add a full cylinder. The benefit of this is that you have a constant gas supply and can change the empty cylinder during the use of gas.