This is a fairly simple trap to make. It is just a trigger mechanism which allows a large rock or log to fall on top of the prey.
In addition to a large rock, you will need three sticks. Two of the sticks should be fairly thick so they can support the rock and provide good contact area.
The trigger stick should be thinner. To make the mechanism more secure and easier to set up, flatten out one end of the trigger stick.
Take the flattened end of the trigger stick and place it between the two other sticks. Line up the whole mechanist so it supports the rock. The trigger stick does not have to touch the rock or any other surface, although it is not a big problem if it does, as long as the mechanism remains sensitive enough.
As always, please keep in mind that these traps, just like all deadfalls are very dangerous. They are as likely to fall on your hand while you are setting them up, as they are to fall on top of some animal. Great care should be taken so a survival situation does not become even worse.