Wikipedia also has an article on this topic.
Danazol is an estrogen inhibitor and a mild androgen/anabolic agent, slowing estrogen production and slightly increasing testosterone. It’s derived from a form of synthetic testosterone.
It acts as both an antiestrogen and a GnRH suppressant. (https://www.drugbank.ca/drugs/DB01406) It reduces estrogen production in the ovaries.
Danazol binds to progesterone and testosterone receptors both bodywide and in the ovaries, and ovarian action may be another key to its mechanism of action. https://www.ncbi.nlm.nih.gov/m/pubmed/6868971/?i=5&from=/2404115/related
The medication normalized serum levels of the sex hormones estradiol, progesterone, and prolactin after prolonged treatment (in those with endometriosis) https://www.ncbi.nlm.nih.gov/m/pubmed/3558772/?i=4&from=/6998453/related .
Danazol has proven effective in treatment of both fibrocystic breasts and endometriosis. It sometimes causes breast atrophy, and due to it decreasing estrogen, I think it could be an effective breast growth blocker for those who want to masculine slightly and prevent further breast growth before starting testosterone. (http://www.sciencedirect.com/science/article/pii/S1658361211701512)
It appears to lower pitch slightly over time as proven by a study from the 1980s https://www.ncbi.nlm.nih.gov/m/pubmed/3966015/.