A torque converter is connected to the engine and receives the horsepower that will be used to move the car down the road. It is full of transmission fluid and this oil is what transmits the power to the transmission gears after modifying torque and engine speed to suit car speed and road conditions. The transmission gears will modify the torque and engine speed still further. When first starting out from a stop, the torque converter swirls the transmission oil around in a way that gives the slower turning output shaft more pulling power than the input from the engine. Think of a tornado, now think of a dog that is going around trying to bite its own tale. If the top of a tornado could be hooked to the bottom of a tornado in a circle so it looked like a donut, and the whole donut was made to spin like the dog, (sorry for mixing my metaphors) you would have some idea of what happens inside a spinning torque converter. All of that transmission fluid swirling around tornado like hits on blades hooked to the output shaft of the spinning torque converter. This swirling causes the fluid to heat up which is energy lost and that means more gasoline. If a car is standing still with the brakes on and the accelerator is pressed down, all the horsepower becomes heat which can overheat the transmission quickly if the car does not move. In stop and go traffic on a hot day, putting the transmission in neutral while stopped will help prevent overheating. The engine can overheat because cooling of the transmission is done by sending transmission fluid to the radiator to be cooled, thus taking away cooling capacity of the radiator away from the engine. If you pull a heavy trailer with your car, you should probably add a special little radiator dedicated to extra cooling of transmission fluid. This is called an auxiliary transmission cooler. On most modern cars, the torque converter has a clutch in it that locks the torque converter to the output shaft so there is no loss of energy in the torque converter at freeway speeds from swirling fluid. The automatic then runs cooler, lasts longer, and gets better gasoline mileage. Sometimes the torque convert clutch fails to disengaged at slow speed and causes the engine to stall when the car comes to a stop.

Behind the torque converter is the second part of an automatic transmission containing gears. Between the torque converter and the part of the transmission containing the gears is the oil pump that circulates high pressure oil for controlling servos and supplying lubrication. In the part of the transmission containing the gears there are clutches and bands which select gears such as reverse, high, intermediate, and low. These are actuated by the servos. The correct selection of gears is controlled by the on board computer in newer cars, and by a governor, throttle position control and vacuum sensor on cars old enough not to have a computer. The modern computer controlled automotive systems are so complicated that some transmission shops are hiring college trained electrical engineers. Also, some transmission mechanics are going into the computer industry. Needless to say, it takes a high salary to retain good transmission mechanics.

When descending long steep hills that would require application of the brakes for much of the downgrade, slow the vehicle to below the speed that the transmission would shift into an upper gear if accelerating hard, and put the transmission into a lower gear. What I am trying to say here is that when you start out on level ground and accelerate hard, an automatic transmission will shift into a higher gear at a predetermined engine speed designed into the transmission and the engine. If you try to shift into a lower gear that is at a higher speed than what that shift point is, the engine will be forced to turn faster than its design and could cause damage. Most modern cars will not shift into a lower gear even if the shift lever is moved there unless the vehicle is brought do a slower speed than the maximum the car is designed for that gear. Once in a lower gear the engine will help slow the vehicle appreciably.

Modern brakes with redundant hydraulic systems will still give about half the braking power if one half of the systems fails, but if the brakes overheat from a long down grade, you can loose your brakes completely. This overheating is called brake fading, and will result in excessive wear of brake parts because of the excess heat produced. If you have been using the brakes hard you might smell something burning and if you are that lucky to smell it before failure, pull over and let things cool down for half an hour. A problem with power brakes is that an increase in braking pressure is overcomed by a small increase of pressure on the brake pedal and fading goes undetected until the car can't be stopped on a steep hill. Accelerating hard to 60 mph and stopping, then doing this again seven times should result in some brake fading. Modern disk brakes are better at resisting fading, but wear will be excessive on a long down grade, and it is still possible to push them to the point of failure. The above also is true about down shifting if you have a standard transmission. A standard transmission will give a little more control and better braking than an automatic. A gasoline engine can add a lot of braking power to the vehicle.