The back side of the jack is connected by coax cable via BNC connectors. If there is no connection to the front (patch plug), the circuit is routed from the input to the output as indicated by the arrows in the top illustration on this slide. When a patch plug is connected in the front, the actuator in the jack routes the signal from the patch plug to the output on the front of the jack. If the jack is terminated, it puts the load on the unused side.