The presentation of the scrotal sack, within a pair of knickers. Arranged to represent a oversized set of vagina lips (labia)
This is then most commonly photographed and sent via text message to as many people as possible.
The best examples either convey a very extreme mess of a vagina, or try to look as close to a vagina as possible. Putting doubt into the viewers mind to whether they are looking at, Balls Flapping around a pair of knickers, or actually just a vile vagina.
(Mike) "ohhh, got a text from tom",
(Charlie) "yer what's that hobo saying",
(Mike) "oh ohhh awwwe, what the fuck is that!"
(Charlie) "where let me look... awwwwww shittt, dude you just got Ball Flapped!"
(Mike) "thats Ball Flapping disgusting. Can't believe Toms Ball Flapping people by text, and on my birthday as well. They do kinda remind me of that chick i muffed out back in Korea though"
(Charlie) "i dunno how to tell you this mike, but that chick may have been Ball Flapping you, you may have just been going down on a guys balls..."
(Mike) *Face of shame*