Ok, i've produced a breakdown for you in order to:

1) Uncerstand what Nolva is/does
2) Understand what an AI is
3) Understand the two types of AI
4) When to use an AI/PCT
5) Knowledge about PCT/SERMs

Nolvadex- increases in the serum levels of LH, FSH, and most importantly, testosterone. And also helps block estrogen. However, it helps your body increase your sperm count because of that. In fact the bad thing about Nolvadex unlike Clomid is that it doesn’t on its own increase sperm count or their motility. However, you still needed it because it does increase your LH and FSH which we need for healthy testis.

Aromatase Inhibitors come in 2 types. Type 1 and Type 2. First Type 1 AI's bind by a process called hydroxylation; this hydroxylation process produces an unbreakable covalent bond between the inhibitor and the enzyme protein. Now the enzyme is permanently blocked even after all of the inhibitor is removed and can only be resumed by new enzyme synthesis. Type 2 Inhibitors on the other hand function all the same in their ability to reduce the binding process of the enzyme and the receptor. Except once the drug is discontinued or the concentration of the drug is sparse enough it is possible for the enzyme to seperate itself from the Inhibitor and eventually will allow renewed competion between the Inhibitor and the Enzyme for the receptor site. Aromasin is a type 1 AI and once it does what it's purpose is we don't need to continue use. Letro and Adex are Type 2 Ai's and the success of those drugs are continigent on the Doses and protocol of which we use them. Once you stop them you expose yourself to an Estrogen rebound. Now having said all of that there are also many other reason to why Aromasin use is beneficial to a Bodybuilder. One is Arimidex/Anastrozole Decreases IGF-1 18% while Aromasin/Exemestane Increases IGF-1 28%. Another is Aromasin is also known to decrease estrogen between 90-95% while boosting Endogenous Testosterone by about 60%, and also help out your free to bound testosterone ratio by lowering levels of Sex Hormone Binding Globulin (SHBG), by about 20% (12)…SHBG is that nasty enzyme that binds to testosterone and renders it useless for building muscle.

SERM- Selective Estrogen Receptor Modulator- are a class of compounds that act on the estrogen receptor. A characteristic that distinguishes these substances from pure receptor agonists and antagonists is that their action is different in various tissues, thereby granting the possibility to selectively inhibit or stimulate estrogen-like action in various tissues. Clomid and Nolvadex are examples of these.
Mode of Action for SERM
Estrogenic compounds span a s
pectrum of activity ranging from:
1) Full agonists (agonistic in all tissues) such as the natural endogenous hormone estrogen
2) Mixed agonists/antagonistics (agonistic in some tissues while antagonist in others) such as tamoxifen (a SERM)
3) Pure antagonists (antagonistic in all tissues) such as fulvestrant (ICI-182780).

You want either adex/asin on cycle at the following doses:
Aromasin @ 12.mg EOD
Arimidex 0.25mg EOD if nipples get sore than 0.5mg EOD if still sore 0.5mg EOD.

References:
Cdaddy7 - (http://www.eroids.com/forum/steroids-qa/pct-anti-estrogens/ais-in-pct-to...)
GS9902 - (http://www.eroids.com/forum/steroids-qa/pct-anti-estrogens/pct-and-aromasin)