Cam Cleat (sometimes called a clam cleat) - Available from boat suppliers (picture above is of a 'Holt Allen' type) A simple device that allows the rope to pass in one direction only - Can be used to pull arms or legs in one direction such as to a beam or corner of a bed and not come slack. Do a Google for "clam cleat" or "cam cleat" for this bit of useful kit. Below Arial Anderssen used it to raise up her arms with the short length of chain making it impossible for her to reach the knot to lower the handcuffs.