This is probably the biggest danger in cock play, as pain may not be a reliable indicator. Tissue that's deprived of blood eventually stops hurting -- which doesn't mean its okay! It may be irreversibly damaged. Any binding is too tight if it cuts into the skin and causes bleeding. Any binding is on too long if it causes the cock or ball sac to become cold or numb, but the third typical sign of circulation problems in limbs that are bound -- loss of colour -- is less reliable in this case. The cock and ball sac normally become engorged with blood during sexual arousal, turning reddish or purplish as a result, and up to a point genital bondage just helps that process along. Circulation could be dangerously impaired by tight bindings before there's much loss of colour. A better sign of impaired circulation is edema, or visible swelling, of soft tissue in the cock or scrotum below or around a binding. While such swelling isn't dangerous in itself (it will normally go down on its own after circulation resumes), whenever it occurs the bonds causing it should be loosened or removed as soon as possible in order to prevent damage. Avoid genital bindings that can't be removed easily even when there's swelling. Tie with a bow-knot or some other knot that will release easily if you pull on the ends -- and be sure to leave the ends long enough to get at. Keep blunt-tipped emergency medical technician's shears (scissors) available to cut bindings in an emergency. Don't use a metal cock ring that's tight when the cock you put it on is soft. It's going to be a lot tighter when that cock gets hard, and you won't be able to take it off until the cock softens again -- which might be a long wait if the ring has caused severe oedema.
To sum up: A good rule of thumb is to loosen tight genital bindings enough to restore full circulation every 20 to 30 minutes even without any signs of a problem. In no case should anything tighter than a comfortable ring or harness be left in place overnight or throughout an extended scene.
If a man is forced to cum while tightly bound around the genitals, the semen may be forced back down the urethra and reabsorbed into the body. Deliberate retrograde ejaculation is a practice of Tantric yoga, and although possibly uncomfortable, there doesn't seem to be any direct medical research on whether it is harmful. Evidence from vasectomies suggests frequent retrograde ejaculations might at worst lower sperm counts or generate antibodies to semen. An isolated incident should be nothing to worry about.
Foreskins can be torn, and if circulation in them is cut off for very long, the tissue won't retract normally and could even die and slough off.
Be gentle with genital piercings: if you rip a ring out, you'll need medical help to repair the damage and avoid a nasty scar, infection, or worse. If a ring does tear out, use an ice pack and local pressure to stop the bleeding (refer to a First Aid Manual) until you can get to a doctor.
A fracture, when an erect penis is struck hard and "breaks" with a loud cracking noise, is an unlikely but potentially serious problem, a medical emergency that requires prompt treatment by a physician. As first aid on the way to the hospital, use ice packs to control bleeding. But if you avoid hard blows to a hard cock, you won't risk the problem in the first place.
The key thing to remember is that levels of tolerance vary enormously. When you're playing with anyone new, always start out lightly with any kind of stimulation to the balls and increase the intensity gradually. With an established partner or in "self-abuse," you can safely begin at a higher level and move faster, but you should still be very sensitive to his (or your own) reactions as you go along. Probably the single most important danger signal in this area is intense and often rapidly increasing pain, so the bottom must be able to let the top know unmistakably when he's had enough. In the vast majority of cases, a bottom whose consciousness is not dulled by alcohol or drugs will have no difficulty in distinguishing between a level of pain that is erotically stimulating and pain that signals real damage.
Much more potentially hazardous is any bondage in which the balls are tied to something else, such as another part of the body or a hook on wall or floor, and might be yanked by a sudden movement: for instance, if you tie a rope or attach a chain between his ankles and his scrotum so that if he tries to move his legs he pulls on his balls. Don't combine this kind of bondage with any other strong stimulation that might cause him to yank on his balls involuntarily, in reaction to pain elsewhere, unless he's otherwise so tightly restrained he cannot move enough to put pressure on them. And never tie someone by the balls to a wall, post, etc. in a standing position without additional support: he could lose his balance or faint and put his whole body-weight on them.
The most common injuries to the balls during genitorture are abrasions (usually from rough-surfaced bindings, such as rawhide or scratchy rope), bruises (usually from slapping or whipping the balls), and tiny cuts (which might happen in any rough play when the scrotum is pulled tight over the balls, or during a shaving), minor injuries best treated with sensible first aid such as cleaning with antiseptic. Bruises generally heal by themselves, though an ice pack can limit swelling. Medical intervention is not usually necessary unless the bruising doesn't fade normally or you suspect an infection.
More serious is a haematoma, which occurs when an injury ruptures larger, deeper blood vessels and a pool or pocket of blood forms between layers of tissue, such as between the scrotum and the balls. The pocket of blood will generally clot in a short time and form a hard mass. Externally, it will appear as a firm, bulging, or swollen area. A small haematoma will usually be reabsorbed without lasting damage. One that is large or keeps growing (because fresh blood keeps accumulating) can "squeeze" adjacent structures, including nerves and blood vessels, reducing circulation to the area and impairing sensation and other functions. If the pressure of a large hematoma is not relieved, permanent damage can result. Prompt medical attention is indicated.
In men who are predisposed to them, minor injuries to the balls can precipitate subsequent swellings, called hydroceles or spermatoceles, in which fluids other than blood build up in the space around the testicles. They can be corrected at one's convenience unless they become infected, in which case prompt treatment is required. Another problem to watch out for is an epididymal cyst; this is not typically caused by trauma but if you notice any unexplained swelling or mass in your partner's scrotum, or your own, do not engage in such play until you know it is harmless or have it corrected. Also, avoid ball bondage entirely with anyone who has a scrotal hernia.
Probably the most serious damage that might occur to the balls during erotic genitorture -- which is not to say that it's likely -- is rupture of a testicle. This is when the outer covering of the ball splits and allows the contents to spill out into the scrotal sac. Besides causing extreme pain, often accompanied by nausea, a ruptured testicle will make the scrotum swell rapidly, and internal bleeding will nearly always create a large haematoma. The ball sac will appear black and blue and be massively enlarged. If this happens, go to an emergency room immediately! The most likely causes are suddenly yanking on the balls or hitting them with a heavy, blunt instrument. Symptoms similar to rupture occur in cases of testicular torsion, which is when the spermatic cords and vessels that suspend the testicle within the scrotum become twisted or kinked, interrupting the normal flow of blood, etc. There will be intense pain, and the scrotum will swell rapidly and be extremely tender to the touch. Surgery must be done within six hours of the onset of pain or the testicle will be lost.
Original from Luisa & the leather family workshop at Kinkfest 2005