What acutally happens when you

Do you mean downshifting, to slow the car ?

Yeah, lets say your in an automatic car and you put it into low gear. I'm wondering why the car decelerates.

Suddenly, the road speed is way higher than the engine speed. Instead of the engine driving the wheels, the wheels are driving the engine, including compression. This overspeeds the engine, by the way. It is possible to float the valves on a large diesel, resulting in valves being in the wide open position when the piston is at top center. This can cause loss of piston and valve, and sometimes the entire engine. If you do it with an automatic, Revival, do be sure you don't drop it into reverse by mistake.

I'm not calling this a bad practice, but you do want to exercise a bit of prudence.

Roger is right, and I wouldn't do it in an automatic unless I absolutely had to.

You can do it in an auto trans, just don't be going too fast. My '93 Mazda MPV has a Hold button on the gear selector that actually holds the tranny in second to allow for engine braking and extra pulling power going up-hill. Actually engine braking in the mountains helps keep the brakes from overheating which can cause brake fade and even worse boiling your brake fluid. When this happens you have no brakes and you better hope your parking/emergency brake isn't frozen/corroded.

Yeah, what the other guy said. The braking is caused by the energy lost in compressing the gas in the engine. It is my understanding that automatics have a mechanism such that you cant select a gear that is to low for your speed. I.e. an attempt to move the gear level into "1" on a highway would be in vain, as would reverse i believe.

The more technical answer is pumping resistance. Forgive me if you know this already... its not my intention to talk down to anyone, but I have no idea what you know about car guts

Your engine is a big air pump fueled by gasoline. As a handy by-product it makes torque which is what moves you along.

Your right foot controls the throttle which is simply an opening to the engine. Its usually a butterfly valve in a round bore. With your foot off the gas the butterfly valve is perpendicular to the flow of air. Only a tiny bit of air sneaks around it, the computer measures how much air there is and injects the proper amount of fuel. In that case you get idle. The engine is making only enough power to keep itself running. When you hit the pedal, the butterfly valve opens parallel to the flow and the computer injects a proportionally greater amount of fuel.

So, say you're on the highway. Let's say you have to use 1/3 throttle opening to make enough power to overcome friction and wind resistance. When you take your foot off the pedal, the throttle slams shut. You now have an air pump with no air. Now the engine is trying to suck air and you have cut it off. The computer is only injecting tiny amounts of fuel so the engine is making much less power than is required to maintain your speed.

Diesels don't have throttles. They ingest atmospheric air every minute. They simply modulate the amount of fuel that gets injected. No foot on the pedal means that the engine is only injecting tiny squirts. Foot to the floor means its injecting lots of fuel. For this reason diesels have a bit less engine braking. On a hatchback its not an issue. For increased engine braking there are three main types of additional braking you might find on heavy duty diesels; 1) a transmission friction brake which is not common since it only works on automatics and tends to cause very high transmission temperatures, 2) a valve brake, which disables the exhaust valves. In a gas engine you cut off its air supply IN with the throttle, so if you prevent the exhaust valves from opening on a diesel, you are cutting off its ability to pump air OUT. 3) an exhaust brake. This is the most common, called Jake brake named for the Jacobs company who pioneered it. It is simply a throttle installed downstream in the exhaust that can close with the same effect as not opening the exhaust valves.