When the comfortable, always present yet never recognized gap between the male knees is forced shut unwillingly (Usually caused by too many people sitting on a bench, couch, bus, etc...). This causes the vacant area for the testicles to do as they please to become occupied by the thighs. This causes the room for one's balls to become blitzed. A few warning signs which often come before a ballroom blitz are "Is someone sitting here?" or "mind if I take a seat?".
Mike: Hey Tim! How's it going? Mind if I sit by you?
Tim: No disrespect Mike, but I'm not just going to let you blitz my ballroom like that.
Football Announcer: And he's going for the ballroom blitz... And he gets the sack! (thought I'd throw in a pun)
1.Song by Glam rock band Sweet, it has been covered several times but most notable by punk band The Damned.
2.Cockney Rhyming Slang for tits.
see also: Blitzkrieg Bop
And the man at the back said Everyone attack
and it turned into a ballroom blitz
And the girl in the corner said
Boy, I wanna warn ya, it'll turn into a Ballroom Blitz