My Mistress did this to me last session. It was her idea. Not something I'd fancied or fantasised about. But she's done it many times before and knew I'd like it. She was so right, as always!
I'd imagined some sort of hot skin-pain, not the deep ball-pain I love. But it was deep ball-pain of some order! She had my balls tied really tight and seperated by a leather cord. An electrode fastened on each one. It hurt so much. She used a setting that pulsed the electricity and it was like someone squeezing my balls hard again and again continuously.
Does that give you some idea?