For example, "I can't spar with my dead brother, not unless he climbs out of his grave."
I looked up the term "not unless" in Merriam-Webster site and it's an idiom that means "only if".
So from my understanding, I think the above sentence means, "I can spar with my dead brother only if he climbs out of his grave."
What if I remove the not preceding unless?
"I can't spar with my dead brother, unless he climbs out of his grave."
So if the dead brother climbs out of his grave, he can spar with him.
What exactly is the difference between "unless" and "not unless" then? They both seem the same to me, at least based on this example.