Computer. Individual Fumble Ratings.

I know this is a 365 forum. Please forgive me.

A buddy and I are playing the computer windows game. Netplay.
We are doing a historical replay.

We turned on the option to use individual fumble ratings.

We both know the old system without that. How that worked.

But this new option..having that turned on…individual player fumbles…
Neither one of us can figure out how this works!

We have both poured through the rules in the computer game manager and we can’t find it. An explanation. How does it come into play…
What dice roll and where does the game trigger a look at the individual player’s fumble rating?

All we know is …the lower the rating for the player the better.

But how does it occur?

Can someone help me here. Explain how that fumble option when turned on works?

Thanks so much in advance