What is the effect on a truck-trailer combination when the parking brake in the towing vehicle is engaged (not in the test position)?

When the parking brake in the towing vehicle of a truck-trailer combination is engaged (and not in the test position), the towing vehicle itself is mechanically braked, meaning its wheels are physically held to prevent movement. However, the trailer is usually not mechanically braked at this point; instead, it is braked through the compressed air system, which applies the service brakes rather than a mechanical parking brake. This setup helps ensure that both the towing vehicle and the trailer stay stationary, but the mechanical parking brake only acts directly on the towing vehicle, while the trailer relies on the air brake system to hold it in place.