hi john m- good answer but one clarification. i have only seen two types of electric overdrives on american cars: the laycock denorman on the 1970s era amc cars(gremlin) which works as you say with the button model and the much more common borg warner type. both were semi automatically actuated at about 28mph by sensing engine rpm by a sensor on the distributor. the difference is the laycock only actuated from high gear. both would shift back to high gear if the speed dropped below 28mph to prevent harmful lugging. the much more common borg warner type had a strange difference which i think was unintended . you had a 3speed manual trans but actually had a 6speed trans. once the overdrive was engaged by a t handle pushed in under the dash, the sensor would sense 28mph and allow the shift into od. the sensor would also actuate a split second and hardly noticeable interruption of the ignition taking the torque load off the trans and allowing the shift into od. interestingly this would give you 6 speeds 1st direct, 1st od, 2d direct, 2d od etc.but you had to manage clutching ,moving the column shift lever and t handling the od cable in an orderly fashion. cumbersome and most drivers didnt do this , only using the od from high gear. one additonal thing is in the borg warner od you had free wheeling until you were in od which would allow you the convenience of shifting up and down while rolling without using the clutch .you only had to step on the clutch when stoppingor going into reverse. i had a 63 and a 65 chevrolet with the od trans which worked exactly like this . im guessing the volvo slide button model worked exactly like that too. the push button volvo since it only allowed od shift from high gear probably didnt . my question toyou is this: i doubt the volvo electric od actually worked by automatically disengaging the clutch to allow the shift into od because you could do the same thing more simply by having a solenoid activated od box behind the trans as in the the borgwarner. any kind of free wheeling adds the safety risk of increasing brake effort to stop the car when you take your foot off the gas. it is surprising that the borg warner ods in most american cars between the early 40s until their demise in 68 worked with all this complication and proneness to trouble. would have been much better to go to a fully shiftable(manual) od as volvo did with the 5speed m47. american cars did this too in the late 70s. i had one of them too in 77 with a dodge which had a 4speedmanual od trans. what do you think? thanks tons oldduke