Single action refers to how you work the airbrush. Each control on a single action airbrush operates a separate spray variable. Simply press down for air and twist out for paint.
The button, or trigger, controls airflow. It’s an on/off device. Twisting the knob starts paint flow and increases paint flow until you twist the knob completely out. The separate paint and air controls make single action airbrushes easier to use than double action airbrushes as you receive a constant and steady paint flow