Aha, good question. To be honest, I'm not quite sure but as far as I know it has vit B6 and zinc and these have symbiotic effects and block DHT (there are studies behind this btw) just like fina. Here are a bunch of studies about zix:
http://advinfoprod.com/hair_formulas_30.htm
The point of zix is to just block DHT locally and not in our body, small amounts of zinc and B6 are enough to affect local tissue DHT but once in the bloodstream these quantities of zinc and B6 are too small to have any serious systemic effect in the body and even when using a topical, a part of the topical will always go into the bloodstream. (that's why I only advise small quantities when someone wants to apply topical fina)
And maybe there's a better explanation somewhere in the zix thread but I don't remember.