, contrary to what the name may imply, is not limited to simply inserting a gerbil up one's anus
. it could potentially involve practically any rodent
that is of *ahem* accomodating size
according to the urban legend and the actual practice, gerbiling is a sexual act known mainly to certain men who derive sexual pleasure anally. for this reason the gerbiling population consists particularly of gay men (it's hypothesized that women, who could potentially have their choice of orifices to work with, could make up a silent minority). the rodent is first placed within a tube (most likely the paper tube found in paper towel or toilet paper rolls). this tube is placed with one end facing the anus. the other end is subjected to the (often repeated) igniting of a match or a lighter. now, this end is not meant to be set on fire; the flame's role is to frighten the poor rodent into delving into the nearest egress; that being the gerbiling man's anal orifice.
what happens after this is pretty much penetration. the goal here can be theorized to be the receiving of sexual pleasure by forcibly placing what would otherwise be considered a "live dildo" into the rectum, thereby stimulating the prostate (the seat of intense sexual nerve endings for many men) through no effort of the one employing said technique. often the case, the gerbil would die of suffocation, as there is a noted lack of breathable oxygen made available in the bowels. the aftermath would involve the defecating of the rodent, live or dead.
felching, however, is the drawing out of freshly issued semen from the vagina or anus, orally (licking or sucking, usually). gerbiling and felching should not be mistaken for the other. although one could theoretically felch out (the remains of) a gerbiled rodent.. but that's just sick.