Bondage is the activity of tying or restraining people using equipment such as chains, cuffs, or collars for mutual erotic pleasure. A number of bondage positions and methods are used in rope bondage and other BDSM activities. Ropes are a common element of these positions, although straps , webbing , chains , hooks , manacles , spreader bars , collars , common furniture, purpose built frames, various gags and monogloves may also be used. The ties and frictions often are variants of Japanese bondage , shibari and kinbaku from where they derived. The ball tie is a bondage position in which a person is bound into a ball position with the knees against the shoulders. Pressing the thighs tightly against the abdomen may restrict breathing.