Breakaway system
Towing
Breakaway switch, breakaway kit, breakaway cable, emergency trailer brake, breakaway battery
A breakaway system is a safety switch with a tethered pin attached to the tow vehicle. If the trailer disconnects, the cable pulls the pin and a small onboard battery applies full power to the trailer's electric brakes, stopping it before it can run free.
Required by law in the US and Canada on trailers with electric brakes, and in most of Europe on trailers above ~1500 kg gross. The breakaway battery must hold the brakes for at least 15 minutes. Test before every trip: pull the pin — the wheels should lock immediately.