Human chorionic gonadotropin. It basically mimics certain hormones and kickstarts glans to restore testicle size such as in the event of when doing testosterone replacement therapy (and the body's production shuts down, and the testicles shrink). It will literally grow completely atrophied peanut size balls back to normal size.
Even with this you're unlikely to see growth
past your natural potential (unless they were already under-size to begin with, with room to grow).
I appreciate the deception of growth etc, it happens to me as well, and I fell into the whole testicle workout routine etc a while back. It just doesn't work. It physically can't. You can't jelq everything - jelqed abs and biceps anyone?
But seriously, you'll just increase the bloodflow there or perhaps even have testosterone fluctuations naturally, or have a buildup of semen after a few days which makes you look a little bigger at times.