A stop loss Trap is used for normally for catching rats etc. The most popular of these stoploss traps are the ones made by a company called Victor. It is very easy to use. You just need to pull the chain down to the clevis in the bend of the long spring where it must stay. Pull the stoploss spring (long wire spring) over it until it rests against the long spring. Stick the curved piece of stiff wire, attached to the chain, through the long spring so that it is over the stoploss spring and hold it down against the long spring. Do not pull against the chain, just set it in a regular way. The basic idea of the trap is that when the animal gets its foot caught in the regular part of the foothold trap, then pulls on the chain, the stoploss spring will swing over and hold its legs.