There are three modes in which you can operate the system, depending on your choices of the two check boxes.

First, check the Use Reservoir box, but leave the Lock Piston box unchecked. This will connect the cylinder to a heat source at its base, which keeps the temperature in the cylinder at the level you set with the temperature slider. Set the temperature of the gas to start. Note the volume and pressure. Now, slide the Position bar to change the volume of the gas. As the volume changes, the pressure changes to compensate, following the Ideal Gas Law.