This is what happens to balls when they’re excited. This video was sped up to show the movement of the testicles while stimulated.
When I made this, I was tired so it would go from soft to hard many times. Notice when it gets soft that the movement of the balls stop. When it gets hard, they start moving again.
At the end, a bead of precum forms.
Enjoy! If you make one of these, let me know!