Refers to a static-electricity-prone outhouse-throne, whereby if you move around too much on the seat in an effort to squeeze out all the
poop, the friction of your clothing rubbing on the
wood/plastic/porcelain
may cause a spark that detonates the accumulated methane in the pit underneath you and explosively goes off
like a cannon, forcibly ejecting you out through the roof of the outhouse, That's why some outhouses are built so tall, to give you somewhat of a "buffer zone" overhead so that hopefully the force of the blast will be sufficiently dissipated before it propels you very
high.
City slicker, looking disgustedly at the super-primitive outhouse at his
country-cousin's out-in-the-
sticks property: Hey, what's with the mattress duct-taped to the ceiling and the clamshell-hinged roof -panels???
Country cousin: Oh, that's just in case the ejection seat goes off unexpectedly... this wire and wrist-strap is to hopefully dissipate static electricity build-up sufficiently, though... you put it on when you're poopin', and take it
ff again when you're done.