TQ lock-up clutch, if your refering to it's function, well, it goes back to the '80's when fuel mileage became a concern in the US. American's preference
for A/T's and demands for better mpg without the european sacrifices of smaller engines & manual transmissions lead to their developement I think.
Torque converters eliminate the need for a clutch due to designed in slipage rates and allow the use of a high axle ratio due to it's torque multiplication effect. The dissadvantge has always been big loses in efficiency due to all the slippage....once a shift's complete the autotrans. itself is as efficient as a manual actually...of the 'fluid coupling' part of the torque converter. To compensate for this, a 'lock-up' clutch was add to the torque converter which allowed a direct mechanic connection between the engine and trans. during 'highway cruise' situations. The problem has always been chosing what was an appropriate 'cruise situation'.
Anyway, @ say 45 or 50 mph, say, at steady throttle, the converter locks to the crankshaft and presto, increased mpg. Hope this helps..........jeffb.