Thank you for the question!
Stunts are meant to be undefined - they're not a specific action you can take. Instead, during the game, you can check off your stunt box to roll a d12 instead of whatever die you'd normally use.
So it's not so much "I am a stunt driver so I can use a d12 to do a sick maneuver", but it's "I'm driving a car and my other skills have worn down, and I really need to get away, so I'm going to check off my Stunt to get a d12 on this roll".
So essentially it can be different every time you use it.
Does that help clear it up? If not, I'd be happy to try to make it less confusing. I appreciate you asking!