This is done using an onboard battery attached to the trailer and a switch system connected to both the trailer and the vehicle. The one stipulation of a breakaway kit is that it does require the trailer to be equipped with electric brakes.
Trailer Break Away Engager SM Breakaway System Kit 12v Battery Charger & Switch
Trailer Break Away Separation Runaway Electric Brake System Kit w/LED Indicator
Trailer Break Away Switch & 12v Battery Box Engager Breakaway System RV Camper
12 Pack Cyclo C10 Heavy Duty Break Away Fast Penetrating Oil & Extended Lube ^
ESCO 1 INCH BREAK AWAY HAND HELD TORQUE WRENCH