From the wikipedia page on 6-oxo:
4-Androstene-3,6,17-trione (4-AT) is a potent irreversible aromatase inhibitor that inhibits estrogen biosynthesis by permanently binding and inactivating aromatase in adipose and peripheral tissue.
Blocking aromatase causes the body to decrease in levels of estradiol, which then results in increase of LH and consequently, testosterone.
A recent United States patent application claims an 88% increase in plasma testosterone levels in men, while decreasing estrogen levels by 11%. The subjects took 300mg orally twice a day for four weeks without taking any other drugs or supplements.
Baylor University made a eight-week study to determine the effects of 300 mg or 600 mg of 6-OXO in resistance trained males. Compared to baseline, free testosterone increased by 90% for 300 mg group and 84% for 600 mg group, respectively. Also dihydrotestosterone and ratio of free testosterone to estradiol increased significantly.