The clover clamp, also referred to as a “butterfly clamp”, is of Japanese design and can provide increasing tension if pulled. The clamp itself is flat, about 5 cm by 10 cm (2 by 4 in) in size. The clamp is applied to the nipple and the spring tension holds the clamp in place. The clamp usually provides a very high pain level, and is usually only used by more advanced users. To increase tension on the nipples, small weights such as fishing line sinkers can be attached to the ends of the clamp. As increased pulling tension is applied to the clamp, it will cause the jaws of the clamp to close tighter, depending on the amount of pull pressure. Another method is to use the clamps to keep a person in one place. If cord is tied from the clamp to a fixed place, the wearer of the clamps cannot move away because the clamps will tighten as they move. Eventually the clamps would be pulled off the nipples but not before considerable pain is felt. If left on for too long can cause nipple irritation.