First of all, driving a race car or a big rig has nothing to do with driving a passenger car. They have nothing to do with each other either. Yes, you should always downshift going down mountains (in an auto too), but you will not stress the brakes at all for stopsigns, which is what this post was originally about.
Second, racers do not downshift to slow down! There are many reasons. It is uncontrollable, inefficient, you don't have revs to spare, you add a lot of wear to expensive driveline parts, and most importantly its not reliable enough. Everybody will miss a shift at some point in a racing situation. If you're at the limit, and you were relying on a downshift to stop the car, you'll eventually end up a part of the wall.
Of course racers do downshift while braking, but this is not to slow down the car. Its to be in the proper gear when they start accelerating out of the corner. The go straight from brakes to gas (smoothly), so the only time they have to change gears is when braking. Hence the heel-to-toe technique.
Racers know the amount of heat their brakes can take. If they're so far gone that they need the engine to stop the car, they might as well give up. That's an extremely unsafe condition.
In an emergency stop with a regular car, do not waste time downshifting. Your regular brakes will stop the car as well as it can be stopped. The brakes have more capacity than the tires, so you don't add anything by braking and downshifting together. Well, you might cause your car to spin out if its RWD.
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