what is lockup?

 

There's a clutch inside the converter that eliminates any slip from the converter. You probably notice a 5th speed on your 4 speed transmission, this is the lockup.

 

From Wikipedia:

Pumping losses within the torque converter reduce efficiency and generate waste heat (referring to how the motor is driven by transmission fluid). In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the pump and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage (in an automatic transmission being fluid driven, there is pretty much always a small amount of slippage until the pump and turbine lock), and virtually no power loss.

 

Lockup was introduced for reasons of MPG and emissions. Thus it unlocks under higher loads than those encounteres when cruising along at 80 mph. If you want a converter that can withstand locking under load, Yank and others can build them, but you'll need a fully-electronically-controlled automatic to utilize such a converter.