Spasming balls....

Wow. That’s hot. Sometimes during oral sex you can tell when a guy is coming, but that... his balls just went into action! No prisoners or sperm swimmers left behind. Great!
 
Some balls retract slowly as the orgasm approaches others like this pair leap into action.
Big dick big cum
Two things: One, you can control some of your balls movement by drawing your lower abdomen in and out. The other is natural (i.e. how your body is designed) to control the temperature for the sperm. When you are in a warm/hot environment your balls will be low in the sac (i.e. hot tub, sauna or sometimes if you have a fever). When it's cold (i.e. a chilly pool) they draw up close to the body for warmth. During orgasm a man's balls pull close to the body to ensure the maximum amount of sperm is pulled into the semen to increase the chance of impregnation. In some vids they almost disappear, but - after ejaculation - relax back into the sac and start making more sperm. The strength of this guy's cumshot was impressive on top of the volume of his load. Who wouldn't want that deep in their belly?