ANIMAL-TRAP
The animal releases the impaling prongs by walking into them.
The trap is staked over a burrow entrance with its ground prongs, positioning a pair of rings approximately the size of the hole. A detent bar hangs across the front ring, holding spring-loaded puncturing prongs in an elevated position. The animal passes through the rings and pushes the detent bar sideways, disengaging the lug from its projection. The spring drives the prongs downward into the animal's neck or body.
“Therefore when the trap is in position as indicated in Fig. 1 and the animal attempts to pass through the rings he will force the member 14 laterally, disengaging the lug 15 from the projection 16, so that the spring 12 will force the prongs 10 downward into the animal.”— Luther M. Steelsmith, from the specification
And the world did not beat a path to his door.