What does Belgian Whistle mean?

Belgian Whistle meaning in Urban Dictionary

A sexual deviation, done between man/woman or man/man.The one companion, presumably usually the one with a sack, opens up within the various other's asshole, and dips his sack to the various other's ass.The other closes asshole, and sack`ed lover draws their balls away from his partners butt, therefore making his ballsack with a healthier coating for the other's feces.