Contemporary torque converters use electro hydraulic Management for that lock-up clutch. The hydraulic circuit which locks/unlocks the torque converter clutch is managed using hydraulic valves. The valves are actuated right or indirectly by solenoids. M (highest performance place): On this position the torque converter hits its highest efficiency as torque https://torque-converter-clutch20741.tinyblogging.com/the-smart-trick-of-torque-converter-clutch-that-no-one-is-discussing-77662446