This is related to a really bad itch on a males scrotum. When the itch is so bad you must place the loose sack skin between the finger and thumb. Once you have a good grab you proceed to pinch and roll the itchy skin between your thumb and finger. This usually results in a boner, from feeling so good, with masturbation to follow.
My balls itch so bad I'm going to have to use the pinch and roll technique to stop it.