Ever catch a big ol catfish on a limb line that didn't have a swivel,only to have the line twisted up to the point of holding the fish on the surface?Well,an otter will do the same thing when caught.(the rolling and twisting up chain in it's case).The swivels for trap use are made of a strap of metal bent into sort of a circular shape with a J-hook at each end to attach to your chain.I shortchain almost all my traps.I have a swivel attached from my trap to trap chain,two links of chain,another swivel,three links of chain,and another swivel that attaches to my anchoring,being it a cable stake,drowner cable,on an extension cable.The latter is for use on cats.