Very early lockup units were purely hydraulic control. You had no control over lockup. Many people complained about lugging, drone, growling once in lockup and the only way to change it slightly was by changing the lockup spring rate. The valve body had a lockup module on it with this spring inside.

Then the electronic lockup was developed to reduce if not eliminate the drone and lugging. It was just a simple lockup solenoid that either opened or closed a vent orifice which moved the lockup valve in the valve body.

Same design was used on the later model 4 speed overdrives, had two solenoids, one for lockup and one for overdrive. Both run by the PCM.

In all cases (3 and 4 speed) the electronic lockup will unlock and lock depending on inputs such as vehicle speed, throttle pressure ( on injected engines from TPS ), etc.