Most thumb cuffs have a serrated edge to prevent removal of the thumbs but the sharp points poke into the flesh causing a lot of discomfort. The ones that are smooth inside have to be closed tight and cut off circulation. Neither work very well. These from China solve the problem. The insides have smooth bulges which form a heart shaped indentation when closed. The security is absolute, these cannot be removed and the gentle protrusions allow circulation without poking the skin. It is the first design I have seen that actually works.