Often shown to an unsuspecting viewer(s) in order to gleefully observe his or her horrified reaction.
(Spoiler Alert! The following reveals elements of the video's "plot")
Sadistic Asshole: "Dude... check out this video of these two hot chicks 'making out'. He! He!".
Said Asshole's victim plays the video "Two Girls One Cup": Idyllic music plays.... Two attractive women ... Standard Lesbian foreplay ... then, at about 0:30 into the video....
Traumatized Victim (horrified and at the brink of nausea): "Are they ... Oh my God, No ... I can't believe ... Are they really, IS SHE EATING THE OTHER CHICK'S POO! AND, OH NO, SQUIRTING IT INTO THE OTHER CHICK'S MOUTH! PLEASE, I CAN'T WATCH ANYMORE!"
Sadistic Asshole: "Oh it gets better. So much better, so much tastier."
... the two women, apparently fed-up (hah! pun intended) with eating each other poop, wash their fecal-feast down with a generous helping of each other's vomit; could there by a more fitting denouement?...
Traumatized Victim (shaking): "Where am I? Its sooo cold in here... My life will never be the same... My whole worldview has been decimated ... Someone, please hold me. I am so scared ..."
Sadistic Asshole: "Ha Ha! Isn't technology wonderful!"