They're not rules. They're physiological responses to stimuli.

Basically, if you cut oxygen off to an area of your body for a prolonged period of time, you take the risk of damaging that area. When you put clothespins on your breasts or clamps on your nipples, you are shutting off the flow of blood in minaturized, individual areas of your body. Therefore, those areas are no longer eceiving oxygen they need to survive.

However, the risks aren't always overwhelming. Some of them are fairly miniscule and I have never seen a published study of the actual effects of leaving clothespins on your nipples for prolonged periods of time. But in the interest of safety, the rules that govern the rest of the body should probably govern these applications as well.

Also, you asked about your legs and if anything could happen from kneeling for a long time. There is a medical condition known as DVT that occurs in patients that have been immobilized by surgery for a long period of time. DVT is a blood clot that forms in the legs and breaks loose and moves through the blood stream and into the lungs or brain causing respiratory failure or stroke. However, in instances of DVT, the time frame is usually measured in days off your feet, and not hours.