I remember (years ago) hitting my balls with a riding crop held in my left hand, me standing, and directing the crop along my thighs until it hit my balls, simply turning quickly my forearm and wrist.

In front of a mirror, I could see the bulge of my left biceps, contracting with every hit.

strength of the hits could be adjusted to induce the appropriate level of pain. it was better to keep to a (relatively!) low level of strength, and to allow a very high number of hits (often more than several hundred) after such a number of hits, although they have been (rather) soft and have not induced high damage, my balls remained sore for hours...

during this time, I was masturbating with the right hand. Sadistic Sara mentionned a story much similar, but in front of an open window...