This emergency system is designed to apply the caravan brakes if the caravan breaks away completely from the tow vehicle. If your van is over 2000kg in ATM you will have this small cable and switch which you connect to your car on hook-up. The switches can only be checked when they are pulled out. From experience a lot of switches break when pulled out. They’re not a high quality item plus the sun deteriorates the switch and causes the plastic to be brittle.
The other main issue is the batteries often run flat. The battery will either be run/ charged off the ‘house’ battery or have its own dedicated battery charged off the car whilst towing. We often find when servicing caravans this battery is flat, so the breakaway system wouldn’t work.