People into BDSM (bondage and sado-machochism) will often use a variety of clamps in a variety of ways. Some are designed to hold the recipient down so various techniques can be administered that are too intense to take without squirming or trying to get away. More often, clamps are applied to the nipples or genitals. Just about any kind of clamp can be used. Typical ones are clothes pins, spring-loaded gluing clamps, hemostats, and vise-grips.
Among the most intense play for women is to apply clamps directly to the nipples, inner labia or even the clitoris. For men, the testicles are a vulnerable target, with ever-increasing pressure to one or both testicles. Testicles are tougher than you might think. They have a very rubbery, strong outer layer that can take a lot of squeezing without rupturing. Generally, the recipient will give up due to pain way before any physical harm can be done. However, one should be careful, of course. Repeated play can desensitize the testicles. Another consideration is sharp edges. Not only can a sharp edge hurt the scrotum, it can potentially disrupt the testicle’s covering, causing a rupture. Finally, stay away from the epididymis, the connections on the back side of the testicles, since they can be torn away, ruptured, or compressed in a way that causes liquid-filled cysts to form.
Interestingly, the instant of removal of clamps can be painful compared to the application. Evidently, the tissue being squeezed gets used to the pressure, and then sensitive to the sudden removal of pressure.