Tunica Tug? Its just like a stretch, except you hold the base of your penis while you do it. Be careful with how much you pull though, it can be deceptively intense.
This is how kingpole describes it:
Take the index and middle fingers and make a ^ pointing down.
Press these against the pubic bone at the base of the shaft and squeeze.
With your other hand, grasp the shaft just behind the glans and as you stretch out, press hard with the ^ back into the pubic bone. Stretch out as hard as you can while pressing back as hard as you can. Squeeze the ^ tight and hold the stretch. Don't release the ^ even if you have to release at the glans. Just re-grip at the glans and stretch again. Hold this stretch as long as you can. You are not stretching the ligs with this exercise, just the tunica. This is a great exercise for Girth.