Prolly nothing, some estrogen is good.
A good example is DHT and hairloss. You have DHT inhibitors and DHT Blockers. Inhibitors will prevent DHT from being made which is good for your hair, but it is bad in that other things use DHT, such as your wang. Now, a blocker, such as nolvadex with estrogen, would block the binding at the site, allowing the hormone to exert it's effects elsewhere. This is why people take spiro over propecia, it is not systemic so it will have fewer sides.
Nolvadex is different in that it is tissue specific and not topical, but you get the idea.