Page 31 states,"Whenever you completely fill a stress track, you don't necessarily have to die, but can try to cling to life by making a test using the defensive stat from the category in which you acquired the stress...."

My question is regarding the "test using the defensive stat".

Can someone explain that roll?

For example, if a PC has incurred 9 physical stress and they have 3 Vitality, what exactly would they roll in order to cling to life?

I think I would have the roll an equal number of dice to the stat. So in your example above I would have them roll 3 dice and if they roll 3 or less then they succeed and take the trauma, if they don't then they die. I don't know if one should actually include negative dice in the roll unless maybe if they have existing trauma's in that group? I'm also interested in what others would do.

Thanks for that idea. It makes sense to me. I'll do that unless someone has a better idea.

It is a normal test of the appropriate defensive stat. You start with one positive die, add any appropriate positive dice (first aid, hardy stamina, help from others, etc.) against a negative dice pool created using appropriate negative dice.

I had this exact same question myself and later came to this conclusion on my own.