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Da hätte ich gerne als Dritter mitgemach

Fantastic!

You can tell the sub is about to nut when his balls contract at 31:11.
Watch closely and you'll actually see the spermatic cord of the cremaster muscle pull each of his balls up.
What you are witnessing is this cremaster muscle at the exact moment of his orgasm contract, yanking each one of his nuts out of his shrunken scrotum into his abdominal cavity until he finishes emptying the combined contents of his vas deferens ducts. Spermatic fluid some % portion of his prostate contents.
Then, once finished with his cumming, the muscle relaxes, so that the balls (like at puberty) are allowed to drop back down into his balls sac!
Each time a man is sexually stimulated and he can feel himself reaching that point of no return, the contractions he feels as he ejaculates are his cremaster muscles of each nut squeezing his balls to release the contents of his vas deferens ducts. How much cum is ejaculated depends on the size of his vas deferens ducts and how much prostatic fluid is included in his ejaculate. It also depends to large degree on the amount of stimulation he has received and for how long. That's why edging produces not only an exponential increase in orgasmic pleasure but also an increase in the amount of ejaculate. It is also why the degree of stimulation is so important in determining the strength of his orgasm. The French have a term for a man's orgasmic contractions! They call them "Ropes"! The more and stronger the Ropes a man can have the better. And consequently how well a lover his partner is.

excellent