This means that there is a large amount of electrical power missing from the torque converter, dissipated as heat. So that you can make improvements to its performance, makers included a lock-up clutch to the torque converter. Together with the a few-passage torque converter, two passages are used for transmission https://torqueconverterclutch82692.theideasblog.com/34468545/torque-converter-clutch-for-dummies