(adsbygoogle = window.adsbygoogle || []).push({});
If you notice the pic next to each achievement, it seems to display the items used/to be used to get the achievement. Take the "Acquire Waffle Weapons" achievement for example. It shows the Wunderwaffe, Ray gun and the monkey bomb.
Now here is the achievement pic for "Activate The Fly Trap" (notice how it says "The" ... meaning there's only one fly trap and it is not one of the normal electric barriers like those in Shi No Numa as stated in some interview)
Now as to what the fly trap actually does might need to wait until we actually use it but from the pics above my best guess would be that it generates a massive electric field fryin any zombie in its proximity. Maybe the characters actually stand underneath/inside it with the field protecting them.
I think it is the pack a punch machine since you have to link all the teleporters to unlock it that think on top recieves the signals or something like that but it might be both