It looks at the number of three point shots attempted and the number of three point shots made by your opponent. I'm not sure exactly what the formula is, but I've seen it bump you to a higher + defense if the other team takes a ton (say, 12) threes, even if they're not making any, and a higher - defense if they take a reasonable amount of threes (say, seven), but only make one or two. It's some combination of these factors.