Maybe I'm thinking of it wrong but it would be difficult to downshift approaching a turn and have the engine just drop to idle. Actually from what some of the other posters said I wouldn't even be able to downshift into 1st while the car is moving; how the heck do you road race with tall gears in only 2nd and 3rd? I'm not trying to be stubborn here and I'm all ears but again I want to be able to upshift and downshift to any gear while moving and keep the engine RPMs matched up to the drivetrain so I can hammer it and have immediate response coming out of turns.
You have the typical misunderstanding of how it works (turning, shifting and breaking during performance driving) and what is going on. This is not bad and I'm not knocking you in any way, it's simply a lack of knowledge.
This is a very simplified version of what is going on
As you approach a turn you are using the brakes to slow the car. At the same time you are setting the car up for the turn and the exit.
When you see or hear the down shifting the driver is not using engine compression to slow the car he is rev matching (getting the car in the proper gear for the turn and exit).
Again
Another simplified example. Lets say one is driving along a straight, flat road at 60mph in 4th. Now lets say the same driver wants to be in 3rd at the same speed without any transition or change in speed. What the driver would do is shift from 4th to 3rd while bringing the rpms up to match what it should be for 3rd gear at 60mph.
It would look something like this in it's simplest form
60mph in 4th > clutch in > rev engine > shift into 3rd > clutch out.
So if you know for example, 60mph in 4th is 3,000 and 60mph in 3rd is 6,000 you would shift out of 4th, rev the engine to 6,000 and shift into 3rd.
The overall goal is maintaining balance. Wild downshifting and breaking in the turn will transfer weight to the outside front tire, unloading the other three (this is why so many 16 year olds park their honda on a tree).
So again it is something like this
.. Slow the car with the brakes as you approach the turn and get the car set up for the turn. When done correctly you will be 100% off the brakes throughout the turn and have the car in a neutral state while maintaing speed with the throttle before acceleration out of the turn.
This is why you hear the "phrase slow in fast out."
