A direct acting actuator responds to an increase in controller output by increasing valve output. For a pneumatic/spring sliding stem valve, an increase in air pressure pushes the diaphragm down, which closes the valve in a "push-down-to-close" (PDTC) configuration or opens it in a "push-down-to-open" (PDTO) setup. The fail-open or fail-close setting is based on the positioning of the return spring and air inlet port relative to the diaphragm, affecting how the valve responds to a loss of air pressure.
