Here's one explanation I had:
The antiestrogen Nolvadex works by blocking the estrogen receptors of the effected body tissue, thereby inhibiting a bonding of estrogens and receptor. It is, however, important to understand that Nolvadex does not prevent the aromatization but only acts as an estrogen antagonist.
Paradoxically, although Clomid is a synthetic estrogen it also works as an anti-estrogen. The reason is that Clomid has only a very low estrogenic effect and thus the stronger estrogens which, for example, form during the aromatization of steroids, are blocked at the receptors. These would include those that develop during the aromatizing of steroids. This does not prevent the steroids from aromatizing but the increased estrogen is mostly deactivated since it cannot attach to the receptors.
Since the antiestrogenic effect of Clomid is lower than those found in Proviron, Nolvadex, and Teslac it is mainly taken as a testosterone stimulant.