A testicle cuff is a ring-shaped device around the scrotum between the body and the testicles such that when closed it does not allow the testicles to pass through it. A common type has two connected cuffs; one for the scrotum and one for the base of the penis. They are one of many different gadgets for restraining the male genitalia.
The choice to have oneself restrained about a sensitive area such as the testicles may stem from several sexual desires. Testicle cuffs can be part of cock and ball torture and other parts of the BDSM culture.
Some men enjoy the feeling of being 'owned', and some people enjoy the feeling of 'owning' their partners. Making the man wear testicle cuffs gives a sense that his sexual organs belong to his partner. Equally, there is a certain level of humiliation inherent in the devices, by which some people are sexually aroused. Finally, the cuffs may form part of a sexual fetish of the wearer or his partner.
Using testicle cuffs can prove extremely painful as the cuffs also restrict the movement of the scrotum itself and the proportions of the scrotum are required to protect the testicles. They can cause the skin around this area to get rather sore if worn for prolonged periods (longer than a few hours). With time, even fluffy ones can cut through the scrotum like cheesewire if too tight, possibly severing the testicles entirely.
Attempting to hang anybody by these cuffs or chain them up to beds could cause internal injuries or rip the testicles or scrotum off.
All the dangers associated with physical restraint apply to the use of testicle cuffs. See also safe, sane and consensual.