The switch has 4 terminals on it, 2 are normally closed, 2 are normally open. The 2 closed terminals connect the ignition switch power to the relay. The 2 normally closed terminals come into play when the kickdown is used. First, it breaks the connection to the relay, which allows for de-activation of the everdrive, second, it enregizes the pull in windigns, which actually pulls the solenoid pawl out of engagement, third, it interupts the ignition for just an instant to unload the driveline, allowing the solenoid pawl to come out easily.
As I said, I've never had to use it, as my engine pulls hard enough to not need it. Also, if I do need a bit more grunt, I'll downshift to 2nd and keep the o/d engaged, it's more gear than 3rd, but not as much as 2nd without overdrive. I figure it's pretty close to having a 4.10 rear gear. It certainly is a good hillclimbing gear, and is fun for driving on curvy roads.
