Why They Move
Small movements of the balls are just the blood flowing through the various veins and arteries in the scrotum. Bigger movements, however, serve many different purposes. Testicles are very temperature sensitive, so the muscles down there move them a lot to keep them comfortable. The dartos muscles (the one that makes the scrotum wrinkly) moves the testicles within the scrotum. The cremaster muscle raises and lowers the scrotum and testes as a whole. During sex, these muscles work together to move a man's balls closer to his body. A man's balls must move upward as part of the ejaculatory process. If you're curious about the workings of the cremaster, you can witness the cremasteric reflex for yourself. When another person lightly strokes a man's inner thigh, the cremaster will move his balls away from this perceived threat. They will relax again once the threat is gone.