system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that when you turn a bunny rabbit on its back, it goes almost completely immobile?

Had a bunny (RIP Turbo) and I used to put him on my lap to rub his belly. 
He loved it, but after a minute he would relax and go completely immobile. Stiff as a board. 
(Worst part is, that's how I found him dead but that's another story....) Any-who...
His eyes moved and he was still breathing, but the only way to fix it was to turn him back over and he'd snap out of it. Any idea why?
gpt
Rabbits have an ability called *tonic immobility*. It's essentially faking death by paralyzing their muscles, dropping their heart rate, and slowing their breathing. For rabbits, this is automatically triggered when flipped onto their backs.

Colloquially, this is known as "trancing" and a lot of people seem to think it's a good thing. However, when a rabbit comes out of tonic immobility, their heart rate and breathing rapidly increase and they get stressed out. I.e., it's a *bad thing* to casually put a rabbit into this state unless doing so for a surgical procedure.