I don't think anyone on this thread has really covered the issues with cock rings very well...
In the penis, most of the blood enters nearer the core of the penis and most of the blood leaves near the surface of the penis. To get/maintain an erection, the body naturally restricts the veins that allow blood to leave the penis, creating a high pressure in the penis, hence, erection! A cock ring further restricts the blood vessels that allow blood to exit the penis (since they are on the surface and can be restricted easily by external, physical pressure) to create an even larger flow differential, creating even more pressure, which means more engorgement, stiffer erections, and a nominally larger size (basically, you're going to get the most out of your penis as is possible with one of these things).
The problem with this is that, like any part of your body, the cells in your penis require oxygen for their respiration, and, without an adequate supply of it, will start to die off (this is called gangrene). Inadequate flow will result in inadequate oxygen supply for any part of the body. One example of inadequate flow causing gangrene is due to frostbite, which happens when extremities are exposed to cold temperatures for prolonged periods of time. The body, attempting to keep the overal body temperature up, will withdraw blood from this area of high heat loss. If that area remains cold for an extended period of time, gangrene (massive cell death) will occur.
In the penis with a cock ring, the lack of flow comes from restricting the outflow of blood from the penis too much. With the flow restricted that much, the blood will stagnate in the penis, leaving it oxygen depleted and rich with waste products (which would be carried away under normal operation). The general rule of thumb is that you shouldn't wear a cock ring for more than 20 minutes of time, because that's when you start running risks of complications due to oxygen depletion in blood.
I hope that explains it to anyone here who's confused...