Here's a post to videos of visible tightening of the belly during orgasm or when getting close. Really interested to see what you all find. I noticed it happens more often using fingers, vibrators and massage tools.

A couple examples:
https://beuretxxx.com/media/xvcf5e390c2669.html
https://fr.pornhub.com/view_video.php?vi...6ca3b5b7ab
https://fr.pornhub.com/view_video.php?vi...9d94a06571 @ 1:00