The simplified route for a car is sensor--->car computer--> actuator. Sensors are not just like 2 sensor, tire sensor but also brake paddle, buttons that intake info. And actuators are like engine, window, sunroof, throttle, oil injector......all the components that receive signal from acr computer and act correspondingly.
Bi-directional test, also called actuation test, is let the diagnostic tool access car computer and send commands, triggering car actuator to response and convey data back to diagnostic tool.
Why is that practical? The bi-directional test lets you test the response of specific system, so that you would know is it malfunctioning. Especially, you are using a diagnostic tool, even when no response from a actuator, you can know for sure if it’s the actuator has problem or car computer has issue with sending command.