[Alex V.]

Lockup converters, at a certain speed, usually only in 3rd or 4th gear - in some early ones, only in 4th gear - receive a hydraulic or electric signal from the transmission at a certain speed after which they lock rigid allowing full power transfer into the transmission without any slippage, hence improving efficiency and decreasing heat. For instance, in an older 3-speed automatic w/lockup TC the lockup point can be felt at a point where it feels like the tranny shifted again, but was already in 3rd gear. Non-lockups have their "stall point" where fluid pressure in the vanes causes them to go stiff, but it isn't a true lock-up and still allows some slippage. Good for torque multiplication, but not beneficial to heat generation or fuel economy.