I'm trying to get my head around the word "or" in that passage, though, which sounds key.
1. "It's POSSIBLE to [step up past D12]"
2. so you CAN automatically stress out your opponent
3. OR you can use a second effect die
This makes it sounds like if you choose Clause 3, you don't also get Clause 2. Like, if I roll . . .
* 23 TOTAL, and D10, D8, D6 - my default effect die is D10
You roll . . .
* 13 TOTAL and D8, D8 - your default effect is D8
I have two levels of overwhelming success, yay! I think I have the following choices:
a) Step my D10 up twice and
stress you out. Booyah! You just got one-shotted. But that's "all" I do: stress you out with one roll.
b) Do my D10 stress to you. ALSO use my available D8 as an effect die of some sort "for free."
What I CAN'T do (I think):
c) Step up my D10 twice, one-shotting you, AND use my available D8 as a free additional effect die. Because AND is not OR, and the rule says "or."
But of course I could be wrong!

So why would I choose (b) instead of (a)? A few possibilities:
* You might already be sitting on D12 stress. I don't NEED to step up my result to stress you out. Now I can do something else with my overwhelming success, like create an asset for myself.
* You might be a D8 or D6 mob or large-scale threat. D10 is already enough to reduce one of your dice. Now I can apply the D8 as additional damage to you.
* I just might like to play with my food a little.
Jim