How do you have to disconnect the air lines when detaching a trailer?

You answered correctly. In Germany, when detaching a trailer, you must first disconnect the emergency line (the red hose coupler) to ensure that the trailer’s brakes are automatically engaged, preventing it from rolling away. If the couplers are of a combined system, both lines can be disconnected at the same time because the system is designed to maintain safety. Disconnecting the service line (yellow hose) first is incorrect because it might cause unintended consequences. Remember: always prioritize safety by ensuring the trailer is secured against movement during detachment.