On a newer tractor, what happens when the driver pulls out the red eight-sided trailer air supply knob?

AAMVA CDL Manual, Section 6.2.3, p. 6-5

Getting push versus pull backwards during coupling or parking can mean driving off with the trailer brakes still applied, or failing to set them on an unattended trailer. Knowing that push supplies air and pull shuts it off and sets the emergency brakes is basic knowledge the Combination Vehicles test expects every driver to have cold.