Not much, it looks just like the 3 speed overdrive unit that chevy put into the 67 2 wheel drive pickups. When it was working it gave you OD in 2nd and 3rd,which was realy nice. One nite as youngsters we were out cruzin and gave it to much gas in reverse and got a little wheel hop (in gravel)and it never worked again. His truck was a 283 2 wheel drive short box.
You clutched it pulled a cable T handle to engage it and that could be done anytime, once engaged you could shift 1-2-3 let your foot off the gas it would engage to OD, if you wanted to down shift to pass it would be like OD 2nd back to 3rd direct than off the gas to OD or something like that.
I think if you put it in I would only engage it on the freeway and take it out in city driving, but then thats probley how it was designed to be used. We tended to keep it engage all the time city and highway and thats was its undoing. But when it was not engage it was perty tuff,he had perfected the dump the clutch back onto the brakes and do the power brakes smoke show.
He and another friend had one in a 57 ford ranch wagon both didn't work at first, both need to have all the electrical connection cleaned real well first probley because of road spray.