It has to have a few things before it will lock-up when it uses the solenoid. As John says it is done mechanically and electronicly when it uses the lock-up solenoid. One of my training books says it started the electronic lock-up/un-lock in 1986. The trans has to shift into second gear so the 1/2 shift valve will send line pressure to the lock-up valve and also past the lock-up solenoid where the pressure is vented until the engine controller (PCM) decides that the vehicle is going fast enough , the eng is over 150 degrees , the PCM knows the eng is not at idle and knows the TPS posistion and the eng vacum is correct for lock-up. Also the trans has to have shifted into third gear so the 2/3 shift valve will send line pressure to the Fail Safe valve and cause it to move over. Once in third gear and the Fail Safe valve moves over and when all the criteria's are met the PCM will ground the lock-up solenoid and cause the line pressure from the 1/2 shift valve to move the lock-up valve over which then lets line pressure from the rear clutch circuit push the switch valve over. When all that happens the line pressure from the rear clutch circuit goes through the moved over lock-up and fail safe valves to the switch valve which moves and redirects line pressure to the converter in the opposite direction which then pushes the lock-up piston in the converter on and locks up the converter clutch. Does not matter if it has what they call a part throttle unlock relay as the relay still powers the lock-up solenoid on or off as the relay is still controlled by the PCM to control when the converter locks or unlocks.
Solenoid off means the line pressure from the 1/2 shift valve vents and will not move the lock-up valve so the converter will not lock-up. When the solenoid is turned on by the PCM then the vent is blocked so pressure builds to move the lock-up valve over and gives you locked up converter. Ron