A sexual deviation, performed between man/woman or man/man.
The one partner, presumably the one with a sack, opens up the other's asshole, and dips his sack into the other's ass.
The other closes asshole, and the sack`ed partner pulls his balls out of his partners ass, thereby leaving his ballsack with a healthy coat of the other's fecal matter.
I performed the Belgian Whistle on my friend's dog last night.
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