There is a wiki on overdrive operation. The overdrive is controlled by a speed governor, the governor closes the contacts in the higher 20's mph. clicks the relay to send power to the solenoid that operates the pawl. a spring has to retract the pawl and it won't move unless torque thru the trans drops. The kick down switch under the gas peday grounds out the ignition coil so the engine dies for a couple seconds and the springs retracts. with a solid state aftermarket system you might have to put in a relay that cuts the power to the ignition when you hit the kickdown switch. Overdrives do not power shift, you have to let up on the gas to shift up and the kickdown switch cuts off the power to let it down shift. With the switch on the shift lever I still had to let up on the gas to up or down shift. Just like on a big truck.