Completely sealed breakaway switches to automatically set trailer brakes in case of accidental trailer breakaway. Required by law in some states. Emergency use only, not to be used to lock brakes when parked or stored. 2010, 50-85-007 and 2005 have reinforced nylon case, weldable metal tab, nylon pin assembly and are rust and corrosion-free. 2009 has all metal parts and a brass pin assembly.