TRAP
The trapped animal's own struggling triggers the trap's second stage.
An animal steps on the treadle, which releases a locking pin from a notch in the supporting arm. A V-shaped leaf spring then snaps the U-shaped clamping jaws together upon the animal's foot or leg. As the animal attempts to move the trap, it pulls the tether chain, which withdraws a delayed-action bar from beneath the jaws and releases a spring-tensioned guard that pivots upward over the treadle. The trap remains fastened to a fixed object by the chain.
“The trapped animal in attempting to move the trap transmits this movement to the chain, which in turn pulls the bar or pin 32 from beneath the terminal ends of the jaws releasing the guard 33. Since the trap is tethered, it cannot be pulled away or lost.”— Asbjorn Hagen, from the specification
And the world did not beat a path to his door.