It holds a zero sum. How it does is by watching it. The zero sum "tries" to nonexist while the Eye of Magnus watches constantly going "Ha you do exist because I see you therefore you can't not exist!"

Why Magnus wanted to preserve a zero sum is beyond me(If he did so intentionally).

So it's the opposite of Numidium? Instead of constantly refuting something, it constantly reaffirms it?

Maybe, instead of holding a Zero-Sum, it is a weapon against deniers. It could be that the Falmer used it whenever the Dwemer tried to deny something out of existence because it would constantly reaffirm the existence of whatever it was being used against.

That's basically what someone said about the Mirror Logicians a while back.

They wield magical mirrors to constantly remind Numidium that it is real, and the reality is real, refusing its refusals.