Down-shifting is an important racing skill that is often poisoned by years of street driving habits. On the street, one typically drives at low RPMs to conserve fuel. Shifting down can be done almost any time, and if one down-shifts early, the engine can be used for deceleration. My father taught me to do this when I was learning to drive. I guess this was a hold-over from the pre-disc brake era when the drum brakes alone weren’t enough stopping power. Why anyone would do this today is beyond me*. The brakes are much better at slowing down the car because they work on all 4 wheels. Brake pads are also one of the least expensive parts of a car and the labor is far less than a clutch.
Down-shitting is the act of downshifting early, releasing the clutch, locking the drive wheels, and crashing. The consequences can be quite dramatic.
On a race track, down-shifting as a form of deceleration is stupid. Assuming your brake bias is set up correctly, engine-braking ruins the braking balance by adding additional braking to the drive wheels. In a FWD vehicle, this isn’t necessarily horrible, it mostly makes the braking distance longer. But in a RWD vehicle, it could cause the rear tires to lock even when going in a straight line. Since a clumsy or slippery clutch pedal could find you dumping the clutch suddenly and going into an uncontrollable spin, downshifts should occur at the end of the braking zone, not the start.
* OK, so there are a few scenarios where engine-braking is useful. When going down a mountain, over-using brake pads can cause them to overheat and fade. And on a slippery surface, adding additional rear brake can shorten stopping distances because there isn’t as much weight transfer to the front tires. If you’re going to use engine-braking, do it because it makes sense in the specific situation, not out of habit.