The governor is just a switch that goes to electrical ground somewhere after 25-30 mph, when you slow down past 25-30 mph the centrifugal force can no longer overcome the governors spring and it opens the circuit to ground, this opens the relay circuitry and disengages the solenoid allowing you to freewheel until the spragg clutch catches when the driveshaft rpms match the engines. I prefer to run a wire from a grounded toggle switch to the governor's terminal on the lockout switch, others run it to the relay but i worry about accidental bumping the switch when the overdrive cable is pulled out.
This switch allows you to decide how soon the overdrive can engage or how long it stays engaged, you can turn on the switch at whatever speed you want under 25-30 mph and it will engage, after that speed its moot as the governor has already closed the circuit. If your going down a hill and od keeps dropping out you can hit the toggle switch and override the governor trying to disengage the od.
I also use the switch a lot when I'm testing the overdrives i rebuild.