I tried pooping intentionally in the past and it just seemed like I was straining and would hurt something, so I stopped and didn't try again for years until recently.
I decided this time to just plain ignore the urge to poop feeling, with no clenching or pushing. It was on a day off at home and I just kept on doing normal stuff around the house. That nagging feeling kept slowly getting more intense. Then later all of the sudden I got an extreme urge feeling and just suddenly lost control and my stomach area just involuntarily contracted and swiftly emptied my bowels into my diaper for 10 seconds. It was quite intense and the release felt really good actually. The sense of relief from that urgency and needing to go feeling felt good also. It happening on it's own with no conscious pushing was a unique good feeling that was unexpected too. Then about ten seconds after it stopped I had that urge feeling again and another contraction and fill.
I was thinking this might happen, but surprisingly I had no erection, actually kind of the opposite. I started involuntarily forcefully peeing along with the contraction and I didn't feel at all like I had to pee before either.
It was really helpless but nice feeling the diaper filling up in the back and front at the same time!
After the two contractions, my diaper was almost completely full front to back and felt really good, but not in an arousing way to me. I just felt really comfortable and content, and had a strange satisfied feeling that I "completed a diaper"
I don't know, it's kind of hard to put into words.
It was the first time I had uncontrollably filled my diaper in less than a minute since being a toddler and it felt way better than I thought it would.
I kept it on and sat down to watch TV for like a half hour. I didn't want to risk getting a rash, so that's as long as I would consider being in a messy diaper.
The cleanup wasn't as terrible as I was expecting, but it still wasn't fun. I will definitely do it in the future in the right conditions, just not very often, as the cleanup is a drag.