In 2010, Mira Bellwether wrote "Fucking Trans Women," the first piece of its kind that outlined the sex lives of trans women as an educational and instructional piece of text. In the zine, Bellwether discusses "muffing," a term that describes the act of pushing the testicle back inside of a male-bodied person's inguinal canal or penetrating the inguinal canals with another object or finger. Before "Fucking Trans Women," there was no term for "muffing" - her zine was and is literally revolutionary. Bellwether expands on several sexual practices, but muffing is so unique and interesting because it is an act that involves penetrating an orifice that most people, even people with a penis, don't know exists.
For those of you thinking, "Wait what?! There are holes above testicles?" Think of it like this: it's the place where balls go when you jump into a cold lake. While we can't see the area with the naked eye, they are located above and behind the scrotum and testicles and covered by flesh.