Basically its a clutch inside the lock-up converter that when applied takes out all normal converter slippage and gives direct drive through the converter in full lock-up. It must unlock at idle or it would be like coming to a stop in a stick car and not pushing the clutch in. As was said when it applies lock-up it basically reverses the fluid flow in the converter so instead of keeping the clutch off it will apply the piston for the lock-up clutch on. The first one Mopar used was in the late 70's I believe and were hydraulic controlled as most newer years are still hydraulicly applied but are controlled by the electronics. Ron