Before I begin I just want to say that if you plan on building this spring spear trap, please be incredibly careful. They are very dangerous and you certainly don’t want to be on the wrong side of one. Use common sense and when the tension is pulled, do NOT stand in front of the spear.
The spear of these traps is propelled at an incredible speed and force. Because of this, do not use the trap anywhere near the beaten path so as to avoid the chances of anybody accidently getting injured.
Basically what happens is that you have a spear attached to a bent sapling and when the trigger is activated, the tension is released and the spear impales whatever is in its path. The US Army Survival Handbook recommends that you use a barbed spear head so as to avoid the chance of simply wounding the animal which would mean that it could simply run away and die somewhere that you can’t find it.