You can find information about the lock-up control for an automatic transmission, including a torque converter. Torque converter does hydraulic connects between engine and gearbox.
Torque converter changes transformation coefficient from 2 to 1 depending on vehicle loading. The transformation coefficient is the relation of the torque ratio on the turbine wheel shaft to the moment on the pump shaft.
In operating modes when the transmission ratio of the transformer is near 1, it works as a hydraulic clutch. The locking clutch of the torque converter connects the pump and turbine wheels. It prevents loss of power by driving at high speed. The locking clutch turns on and off when the direction of the working fluid flow inside the torque converter cavity change.
Engaging the torque converter lock-up clutch
When the vehicle is moving at about 50 km/h, the torque converter clutch control valve applies pressure from the valve to the rear of the clutch and the piston is pressed against the housing. The clutch is activated and the moment from the crankshaft is transmitted to the input shaft of the gearbox, past the torque converter.
Turning off the torque converter lock-up clutch
As the vehicle speed decreases, the lock-up release valve pressure decreases and the torque converter clutch control valve applies pressure to the front of the lock-up clutch and the clutch disengages.