A German asks a prostitute for sex and she tells him it's 20 dollars. "Fine" he says, "but I'm a bit kinky". She agrees that this is OK as long as he doesn't do anything violent.
They get back to her flat and he gets out four big springs attached to some straps. "I want you to put one of these on each elbow and one on each knee" he asks.
The prostitute is worried that she's getting into something a bit heavy, but she goes along with his request. Then she is told to get down on all fours, naked, in front of him which she does grudgingly. Then he asks her to start bouncing up and down on the springs and finally he takes a duck call whistle from his pocket. "Blow on this while I'm shagging you" he tells her.
So he's banging away at her from behind while she's bouncing on the springs blowing the duck whistle. Suddenly she starts to enjoy the shagging, so much so in fact that she experiences the most fantastic orgasm she's ever had.
After they've finished she says "Wow, that was the most fantastic sex I've had in 25 years on the game, how the hell did you make it so good?"
"Ah," the German replies. "Foursprung Duck Technique".