Gargle : to hold (a liquid or in this case a solid) in the mouth or throat and agitate with air from the lungs
Gargling Balls: when a person metaphorically gargle (aguh aguh aguh...) the balls of another person with their mouth because he is that person's personal servant or bitch.
Joe goes out of his way to make Bills day, therefore Joe gargles Bills balls.
Eric does what Tim wants, therefore Eric gargles Tim's balls.
When one person performs the act of putting another's testicle's in someone else's mouth. You also have to make the sound effect. Like your choking.
Hey Emma, wanna be gargling balls tonight?
Yeah sure Ben, love to ;)