The levers are pulled, and two walls drop down, sealing any enemy where it can be taken care of by marksdwarves.
Since constructed walls disappear when dropped, you have to use natural ones. Usually, you'd have to mine a new trap every time, but this one creates new walls right on the supports.
The only way a forgotten beast could avoid the trap would be if it swam through the magma supply, and the only way one could escape would be if it could go through fortifications.
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