This is a type of temperature play. The actual amount of time you can leave clamps in place varies depending on the person and on how tight the clamps are. Just go careful, some can be too strong and some have rather sharp edges. One advantage of this technique is that because the nipple isn't being clamped, and so long as you don't wind the cord tightly, there won't be a blood circulation problem and the cord can be left in place for extended periods. Another type of alligator nipple clamp.