From the diagram on slide 2, you will notice that the valve shaft runs the length of the valve body connecting the control knob and the pad seal together.
When you turn the control knob in a counter clockwise rotation, the holding washer pulls the valve shaft outwards from the valve body, this lifts the pad seal and then allows steam from the boiler
through to the steam arm. The more the pad seal is lifted, the more steam is allowed out of the steam nozzle.
After steaming is complete, turning the control knob clockwise causes the tension spring to expand and force the pad seal to close the valve.