What is the function for the lock up/shift solenoid on a 1999 Honda Accord Transmission 4 cyl?

In an automatic, the torque converter acts sort of like a clutch in a manual transmission. However, there is always some slippage in a torque converter which results in lost power. The lock-up solenoid locks the input to the output of a torque converter, like a manual clutch, thus eliminating the slippage and improving your mileage. Generally, the lock-up only engages when you are driving at a steady speed. It disengages whenever you either press down or let up on the accelerator.