Well so it's true that you want to minimize downchanging while turning but sometimes it can't be helped. If you're driving a car that has a rev-matching (auto-blipping) gearbox like the one in the Camaro, you can get away with it a lot more easily.
A good example of a place where in this car I sometimes make a very late downchange is through Brooklands at Silverstone. It's the long decreasing radius left hander at the end of the first long straight. Because it's a decreasing radius you just keep going slower as you progress through the corner, and sometimes I want to go down to 2nd there. It never causes me a problem.
OTOH if I have to rev-match it myself, by blipping, then it's riskier. If I get it a little wrong and am at the lateral limit, it may chirp the rear wheels enough to put me sideways.
Anyway this might be more than you're looking for but my advice is to experiment with it. There's no getting around the need for the rear wheel speed to match the road speed though. If the clutch is engaged, the rear wheels are connected to the engine, and the engine will only allow them to rotate at whatever speed it's turning (modified by the gearing). So what happens when you should be in 4th and downshift to 2nd is that the engine can't turn fast enough to allow the wheels to rotate at road speed. It isn't quite that simple but that's one way to think of it.
Really the engine can turn much faster than its "redline," and a rev limiter cannot stop it from doing so when you downshift inappropriately like this. I haven't tried this in rF2 but in a real car the engine will, given enough time, in fact speed up to whatever the road speed requires. If that means you zing it way above redline then so be it, and you've probably done some damage if not grenaded it.
Understanding that, consider the engine in the Camaro. 7.9 liters if I'm correct, with a 6500 rpm redline. That's a big, slow engine, and so when you try to accelerate it, it doesn't happen quickly. The result is just nearly locked up rear wheels as the engine can't keep up.
The fact that it has such a narrow rev range, relatively speaking, is probably part of what you're having trouble adjusting to. In most race cars, and most cars in rF2, there's a much wider rev range to play in.
