Hi, it's not about pain at all.
It's something you learn to do.
It took me about 3 months, working up from a dick, to get to a hand.
The first experiences were very much about the sense of fullness and sense of achievement of being filled.
As time went by I started to enjoy the sensation of motion.
At first it was the prostate area that was pleasurable but during one session something extraordinary happened.
I had a full body orgasm.
During that session I think I had 8 full body orgasms.
It was an amazingly fulfilling experience and gives sex a different dimension.
I find normal male climaxes rather boring and much prefer the other form.
The orgasm seems to be triggered when the hand is well inside my bottom. There seems to be a sensitive area.
Hope this helps.