The extra wrinkly folds as obtained from having such conditions as Hound Dog Syndrome (HDS) are wrapped around the penis and used as a fuck toy. The penis is thrust into the surrounding flab of skin, usually with some form of lubrication, either saliva or artificial lube.
"That fuckingex-fatty Sarah was giving me terrible head, so I flap trapped the fuck out of her."
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi