When one attempts to release a short loud collection of methane gas, otherwise known as a fart, but releases a concentrated sphere of fecal matter into ones underwear. The collection of feces is usually leathery in texture, thus preventing excessive smearing, but it will stain.
The difference between a buttplosion and a shart is that it is more than possible for a shart to leak out quietly and discreetly. A buttplosion is incredibly sonorous, and cannot be done discreetly as it is a powerful, full-force anal gust that pushes the turd into the pants.
After a buttplosion it is imperative that the victim behave nonchalantly. It is a guarantee that the buttplosion was heard, but the crowd will only think the victim farted. Laugh at yourself, or pretend it wasn't you; but escape the premises as soon as possible, because the resulting anal waste will cause a foul odor to accumulate around the company you're keeping. Go to a safe and secure location, get some rubber gloves, and go through with the disgusting and sickening act of cleaning up the large bits of poop that has most assuredly spread copiously in your underwear. It is probably best that that the soiled underwear be permanently discarded after the incident, for there will undoubtedly be stains.
-Did you see Bill run out of here, what was that about?
-He farted and I think it was a buttplosion. He seemed pretty worried and scared.