I once had this guy put some anal beads that had some reverberating balls in it. Like with any movement the inside kept bouncing around like water in a bucket.
Probably would be difficult because of gravity. Some guys are stomach riders, some are back, some are side.
It sounded like an interesting idea. What comes to my mind is that we can use the steel spheres containing a spring mechanism that the ancient Chinese nobles moved in their hands for hand massage. If you hold these spheres in your hand and shake them, the spring inside will vibrate, make a ringing sound and shake on its own. I think that after we place these balls behind our back, they will vibrate inside us with a slight body shake. I'm sure this will gently vibrate and stimulate our prostate. I'll try this one day. I recommend you to try it too. /Sorry for my English. Translate is the translation/