The throttle valve cable (TV cable) is present in all modern
cars with
automatic transmissions. Its purpose is to allow for upshifting appropriate to the
driver's driving style and
downshifting when rapid
acceleration is necessary.
The TV cable is attached on one end to the throttle linkage and on the other end to the transmission. As the throttle (gas pedal) is depressed, the cable is pulled, causing progressively later upshifts at higher throttle positions. Additionally, when the cable is pulled a certain amount (ideally, when the pedal is completely depressed), the transmission, if possible, is made to downshift.
The reason for this is as follows. The engine produces the most torque in the higher rpms (up to a limit, obviously; there is a point known as a "sweet spot" during which you can feel your engine providing the maximum torque, past which it decreases again), resulting in higher acceleration. Furthermore, the lower gears produce more torque and less speed, whereas the higher gears do the opposite.
Therefore, the fastest way to accelerate is to stay in each gear until right after your sweet spot, and then upshift. Additionally, if you're going at a certain speed and wish to suddenly accelerate, the best way to do so is to downshift. At some point in the evolution of the automatic transmission, car manufacturers realized that the further down your gas pedal is, the faster you probably want to accelerate. Since accelerating faster means upshifting later, the throttle valve cable makes the transmission upshift later the further it is pulled. And, the car manufacturers realized that if you floor it, you want speed, fast. So, if it gets pulled to a certain length, it makes the transmission downshift (unless, of course, you're going very fast in the highest gear, in which case it has no effect on the transmission).
Having a maladjusted TV cable is not a good thing. Not only will you upshift either early or late, but you will also, if the cable is too slack, eventually burn your transmission clutches. To adjust the TV cable, you will need an assistant. Open your hood (engine off) and locate the TV cable. It will probably be near your carburetor or fuel injection system; make your assistant fiddle with the gas and watch for what moves. On older cars, you may need to remove the air filter to get acceess to it. Now tell your assistant to fully press the gas. The exact method for adjusting is diferent from car to car, but the basic idea is there will be a cable being pulled out of a sleeve; there will be some mark on the cable; this mark should be at the edge of the sleeve when the accelerator is fully pressed.
Once you have adjusted your TV cable, drive around a bit to make sure that it downshifts when you want it to and upshifts at the times you think it should. It's hard to get the adjustment right the first time - the best idea is to start high (i.e., downshift early) and move lower and lower.