For those interested see below a detailed explanation from Lord Kunos himself.
"Steer assist is a variable power steering. It doesnt work on speed but it works on the amount of force. It's basically a gamma function, I use gamma functions a lot in AC.
So let's say you have a FF input from -1 to 1 (it's the job of the ffmult to get the actual torque and scale it down to a normalized -1 , 1 range)... the resulting FF after the steer assist will be:
assist_ff=input_ff^steer_assist
The job of steer assist is to allow the driver to feel the steer "full" at low speed with cars with heavy downforce without killing his arms if the car is going through a corner at 200kmh. The rule for steer assist is very simple:
Are you doing a F1/LMS or other VERY high downforce car designed in the last 10/15 years?
yes ? experiment with steer assist value below 1 only AFTER you've tuned the ffmult to give you a good range of forces in the -1..1 area
no? steer assist is 1"
From Stefano Castillo