system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you take a particularly harsh bowel movement, why do you get that brief rush of pressure to your head?
gpt
You've got a very long nerve in your body that runs all the way from your brain down to where you poop.  When you bear down (push really hard to get the poop out) you can stimulate that nerve.

That nerve happens to be in charge of some very important stuff in your body, like telling your heart to slow down, telling your blood vessels to dilate (reduce your blood pressure) ect.  So when you stimulate that nerve by pushing really hard on a big poop you sometimes make it cause those things when you don't need them to happen.

So suddenly your blood pressure goes down and you feel it first in your head (cause gravity, and your brain is important).  This sometimes causes people to pass out.

This is the Vagus nerve and the term often used is a "vasovagal response" meaning your vessels responding to your vagus nerve. Or even "vasovagal syncope" which means temporary loss of consciousness that resolves quickly due to vagus nerve stimulation.