If it's too soft, the car squats and the IRS changes angle and the tyre contact patch is less, therefore less traction.
But too hard in suspension the tyre (sidewall) needs to be the suspension and take the motion, if you don't have enough sidewall, this will lead to wheelspin.
It is an artform, i don't know much about it but i'm trying.
Benny i've showed you a few times how to launch a powerful IRS car before a few times haven't I?