I like this explanation the best:
Femara is one of the most powerful anti-estrogen and aromatase inhibitors available. It is used by men to eliminate water retention and bloating for a more ripped and shredded look. The body has a tendency to convert excess testosterone into estrogen (aromatization) which can cause side effects to bodybuilders on heavy testosterone and prohormone supplementation. Such estrogenic side effects can include highly sensitive and enlarged nipples (gynecomastia), excessive water retention and skin rashes. Femara works by blocking the production of the aromatase enzyme to estrogen and by doing so eliminates much of the bloat that goes with pro-hormone or steroid cycles. Femara is great for keeping excess water weight down when you are taking heavy supplements, helps to prevent gyno and enlarged breasts for men and can even be used to help get your natural testosterone levels back to normal....
I can tell you from experience it is the most effective anti-e I have ever used and I will be hypothetically ordering more as it has been effective in my post contest recovery....