The pressure switch will be closed (passing electricity) if the tank air pressure is below the pressure switch low pressure set point. If the air pressure in the compressor tank is higher than the pressure switch low pressure set point, the pressure switch will be open (not passing electricity) and the power stops there.

Inside an air compressor pressure switch

In the photo above you see two views of the same pressure switch. The image on the right has the cover on, and on the left, the cover off. Note that there are two wire-ways under the floor of the pressure switch. One of these is to bring the power cord from the wall plug through the wire-way and up to the terminals on the pressure switch, and the other is to allow the wires from the switch back out of the switch and up to the motor circuit.

The wire terminals themselves are usually marked with LINE – which is the terminals for the power supply from the wall plug, and the other side is noted as LOAD or MOTOR, and the motor wires are attached to these terminals.

On the right side of the left hand image above, you can see the screw to which the ground wire is attached.