The pictures tell the story!  The story goes that the Americans supplied some old WW1 surplus Harrington and Richardson handcuffs to the Chiang Kai Chek regime in their struggle with Mao Tse-Tung and when they ran out the Chinese copied them and the rest is history. These single locking handcuffs are ingeniously designed with a key hole which is only exposed when the chain is canted at right angles. The key is like a female spanner with an unusual triangular shape (see picture) which engages a good inch inside. Very difficult to pick and impossible to shim as the bow, when engaged, drives a small wheel, not a ratchet. 

This triple version can principally be used for prisoner transfers and attaching the two handcuffs to a secure object such as a radiator or wall hook, all you need is some imagination.