system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come you can be at a point where you have to poop SO BAD it is almost completely unbearable, but after a few excruciating minutes that feeling just 'goes away'? Does the poop literally go back up the intestine? What happens?

Sorry to be vile - legit question, just not sure how to phrase!
gpt
When poop initially moves into the rectum, it triggers a reflex where the internal muscles of the anus and rectum relax, allowing pooping to proceed.  When this happens, the anus becomes the last barrier between your poop and the great outdoors, and you feel that pressure.

If you hold back for a while, the relaxation reflex fades and the internal muscles re-engage.  This takes some of the pressure off the anus and the poop does move back up into the sigmoid colon a little bit.  If you try to poop after holding it for a while, you may notice that it takes a bit longer to get things started, since you have to force all those muscles to relax again.

WARNING: As poop sits in your colon while you're holding it in, your body will continue to compress and extract water from it, resulting in much harder and potentially painful bowel movements.  Over time, this can contribute to hemorrhoids, anal fissures, and other unpleasant medical problems.

TL;DR - Don't get in the habit of holding your poop more than necessary.