Nipple and Clit Clamps can be a lot of fun and certainly enhance sexual activity. S&M Pain sluts absolutely love them. Other submissives find them very punishing and thus a discipline type item. Nipple clamps will cause a significant amount of pain when first applied. After a short bit the pain will dull out as the area where they are applied becomes somewhat numb because the pinching cuts off  or significantly restricts blood flow to that area. If blood flow is cut off completely over a prolonged period of time, the tissue will actually began to die. When the nipple clamps are removed the pain returns a blood flow is restored to the area. With the increased blood flow, sensitivity is heightened, meaning that the since of touch to an area that has been clamped is much more intense. Also, after nipple clamps have been on and the pain has dulled, a slight tug on the clamps can cause the blood to try to force itself into the area and therefore causing the pain that was non-existent to return as the clamp is tugged. The harder the tug, the more significant the pain.

For someone who sexually enjoys the pain, the only really question is how much of a tug is enough. Another common practice is to add weights to the nipple clamps. Usually only a few ounces of weight is needed, like 4 to 10 ounces of weight. If the submissive or slaves moves or jerks, the weight swings causing a tug on the nipple clamp. This is sort of a predicament torture for the submissive because they must only let themselves in relation to how much pain they can endure and by moving to much, they may cause themselves to have to endure more pain than they work like. It's a very wicked and delicious thing. 

Because prolonged clamping can cause tissue damage, most people will tell you that you should never leave them on for more than 20-30 minutes, however I personally have been adorned with them for a much much longer period of time. The key is, it really depend on the type of clamps being used. For instance, alligator style or tweezer style nipple clamps can very aggressively clamp down on the tissue, pretty much denying all blood flow and thus should not be left on for very long. On the other hand, Japanese Clover Clamps don't pinch too hard when just clamping the tissue, but if pulled, clamp down hard and hard as you pull. Because they only clamp down enough to hold, blood flow is restricted but not completely cut off and thus it is safer to leave them on much longer. Also you will notice that many of the alligator style clamps sold have adjustable pressure screws. This allows you to adjust it so that the clamping action is just strong enough to keep it on, but some restricted blood flow would be allowed, thus again making them safer for longer term use.