This trap works when the prey touches( or steps on it human wise) the trigger makes the dead tree that was stable and laying on the living tree around it become of balence, causing it to come crashing down on the desired prey.
This trap works when a sharp object , forexample in the story it was a sharpened piece of wood, is placed upwards in a soft spot that is covered. Covering it will make it harder to detect the trap.
In order for this trap to work, you must tie a saplian (flismy branch or so) to a bigger tree. Next, you attach a knife to to the set up. Then once your prey/target comes across the trip wire or trigger, the knife will fall down- striking your target.