Used on surge activated brake systems.  Unlike free backing drum brakes, disc brakes generate full braking power in reverse.  Attach this switch on your trailer brake actuator and connect the wiring to the reverse light of your tow vehicle and your trailer automatically freewheels when tow vehicle is put in reverse.

• Single wire 12 Volt DC - In-line - Normally open.  Solenoid valve closes when activated
• Inlet & outlet 1/8" NPTF 
• Can be wired independently or adapted to trailer wiring harness