I noticed that when I downshift hard into a corner, my car starts to fishtail. Can anyone explain why? And what I can do to stop it?

accelerate gently while the clutch is engaged in aim to have enough revs when the clutch will disengage . The same appear with real cars (rwd mainly), especially with NA engine of consequent displacement ... For the moment there is no proper automated gearbox doing the job for you for the curent cars.

I maybe wrong but I tend to remember that at a time in the past there was the throttle blip as an option ...

I'm always blipping while braking. It's a habit for me. Just use throttle when downshifting and you won't have problems anymore.

If you are using manual clutch, try to lift your left foot slower.

Anyway, in addition to what Flotch said, you can wait until your RPMs are lower to reduce the impact of downshifting. Sometimes shifting 2 or even 3 gears in a row is better than maxing the RPMs in each downshift. It varies on car to car, driving style and even track surface and race line.

It's called shift lock. It's a way of drifting but costs many gearboxes.

Watch this vid, very good explanation why you should do the heel&toe

Quote from Flotch :

I maybe wrong but I tend to remember that at a time in the past there was the throttle blip as an option ...

Version 'X' was the last one to have this option, I am glad that devs removed it, this forced me to teach how to do the heel&toe.

Quote from kars19 :

...Version 'X' was the last one to have this option, I am glad that devs removed it, this forced me to teach how to do the heel&toe.

good memory . The only concern I see is that AI do not do it (I thought they did it in the past) ... and it participates to the lack of efficiency they may have...

Quote from kars19 :

Watch this vid, very good explanation why you should do the heel&toe
https://www.youtube.com/watch?v=cPj9XXW25GA

Also https://www.youtube.com/watch?v=AMz6zPavWEU&feature=player_detailpage If you aren't drifting, just do not do and avoid anything in the vid.

With shift lock you can initiate slide, but it's stupid to do so, bad for gearbox and clutch, and IRL drifters don't use shiftlock, it's not that effective and very inaccurate, 90% of the time it's the E brake

This is very annoying to me, as my Logitech Driving Force pedals don't support to keep both pedals down at the same time in LFS.

Quote from Jesse Kyytinen :

This is very annoying to me, as my Logitech Driving Force pedals don't support to keep both pedals down at the same time in LFS.

Is there not an option in the profiler to turn off combined pedals? I know the DFGT does.

When braking with a car like XRR, you should apply gas when shifting down (just by a tiny bit so the RPM's go up), should fix the problem ez pz.

I play with gamepad so i have to lift brake between downshifting... You can try that maybe?