The overdrive used in the old trucks was an electrically operated unit, essentially a 2 speed planetary gearset. In the original applications, it was on when the ignition switch was on, the activation was controlled by both the governor (cut in about 28 mph) and a lockout cable that would prvent engagement when not desired. Here's a link to the manual describing the operation.
http://chevy.tocmp.com/1955truck2shop/55ctsm0716.htm I have mine on a separate switch, so it's only on when I need it.
