Hey.
I don't have a how or why, but I guess it has to do with the built in features found on the IRL rally cars.
My test was, get the fastest RWD cars in Dirt 2.0 and down shifted from 4th to 1st on tarmac. As seen in the video of Karik134. The engine rpm, remains very high and there is no engine breaking.
But since I only got my wheel set up past weekend. My skills are lacking and I crashed. When I wanted to get going again, I noticed it didn't matter if I depressed the clutch pedal. The car accelerted aither way.
Maybe these cars have a DSG or a sequential with a robotized clutch. That's the part I don't know. But it got me thinking.
So when I jumped in the RWD Sierra from the H3 series, I suddenly did have a clutch I could oparate. I could depress the clutchpedal mid corner and would lose RWD...
First good sign!
When I made it up to 5th and shifted back into 1st, guess what the car did!?
The rear wheels locked up, making the car uncontrolable and spinning me of the beautiful Spanish roads down a 30meter drop
off course I kept my cool.
So there you have it, it's not the software or a setting. It's litterly because some cars are designed to handdle this IRL and Codemasters decided put that into the game.
TL,DR: Try an old car with a manual gearbox.