The design can be used as a trocar, or it could otherwise be attached to a drill so that the drill doesn't damage anything directly behind it. My own design but not really my own idea.
Requirements: a 40mm*10mm spring, a 16mm m3 self tapping countersunk screw, 2 6mm m3 hex bolts and nuts.
How to use it:
push in the cap so that the flaps of the inner pin are beyond the flaps of the body. Push the pointy end against something so that the diamond shaped piece expands an the 2 pairs of flaps catch on each other. when the pointy end has pierced through something it should automatically retract
The author hasn't provided the model origin yet.