I have experienced this as well, but only once so far. An ex (who I've continued to play with from time to time) was abusing my balls for about 20 minutes when she asked how I wanted to finish. I suggested that instead of having sex she try and get my off by going harder, while i was laying down. She had one hand gripping them, while she hit them with her knee forcefully for a few minutes till I ejaculated. It was incredibly intense.

I had suspected that it was possible but had never tried before, and was pretty surprised myself.