Testosterone promotes nitrogen retention in the muscle , and the more nitrogen the muscles holds the more protein the muscle stores, and the bigger the muscle gets. Testosterone can also increase the levels of another anabolic hormone, IGF-1, in muscle tissue . IGF-1 is, alone, highly anabolic and can promote muscle growth. It is responsible for much of the anabolic activity of Growth Hormone (GH). IGF-1 is also one of the few hormones positively correlated with both muscle cell hyperplasia and hyperphasia (this means it both creates more muscle fibers as well as bigger fibers). All of this leads me to speculate that for pure mass, IGF-1, GH, and Testosterone would be a very effective combination. Testosterone also has the amazing ability to increase the activity of satellite cells. These cells play a very active role in repairing damaged muscle. Testosterone also binds to the androgen receptor (A.R.) to promote all of the A.R dependant mechanisms for muscle gain and fat loss , but clearly, as we've seen, this isn't the only mechanism by which it promotes growth.
Testosterone has a profound ability to protect your hard earned muscle from the catabolic (muscle wasting) glucocorticoid hormones , and increase red blood cell production , and as you may know, a higher RBC count may improve endurance via better oxygenated blood. The former trait increases nitrogen retention and muscle building while the latter can improve recovery from strenuous physical activity, as well as increase endurance and tolerance to strenuous exercise.
Testosterone is the prime male androgen in the body, and as such still the best possible mass builder in the world. It has a high risk of side-effects because it readily converts to a more androgenic form (DHT) in androgen responsive tissues and forms estrogen quite easily. But these characteristics also provide it with its extreme anabolic tendencies. On the one hand estrogen increases growth hormone output, glucose utilization, improves immunity and upgrades the androgen receptor, while on the other hand a testosterone/DHT combination is extremely potent at activating the androgen receptor and eliciting major strength and size gains. While not always the most visually appealing result, there is no steroid on earth that packs on mass like testosterone does.
300-400 mg per week is minumum amount for good muscle growth. More advanced cycles can contain 500-600 mg per week. Maximum dosages can be as high as 1000-1500 mg per week.