This is another memory of one of the episodes Mark and I had during our prolonged mutual masturbation.
During the school year, I would meet Mark every day at his house after school for a jack off session. After we got used to it, he would be naked when I arrived, lying in bed lubed up slowly jacking.
I had a key, so he didn't have to answer the door. The first time I saw him this way, he was near to cumming when I arrived, and I noticed that his balls were tight up against his pelvic floor spread apart. He was lying on his back with his knees bent and spread apart, so I had a clear view of his behind, anus, balls and his fist enclosed penis.
I watched fascinated as he got closer to orgasm, and as his excitement increased, his anus seemed to open and close, and his balls looked like they were alongside his shaft instead of under it. When he came, his pelvis looked rigid and hard as a board, and his balls seemed to disappear into his abdomen. By the time he had calmed down, I was nude and lubed up ready to jack.
He asked me to lie down in bed while he watched me as I had watched him. I agreed. When I had finished, we described what we had seen to each other. We both agreed that as we got closer to orgasm our balls retracted, our anus's opened and closed, and just as we shot off, our balls were well up in sockets behind our pubic hair.