Purrito

as a non-cat-owner i have a question -> why a harness works like that for you? is it because the cat hates it to wear it? is it because of fear because she is not used to it? or is it because of training (she learns that harness = i'm should be nice because i go to the vet now)? or maybe because a reflex you usually know from baby cats when their mother bites them in the neck (without hurting them offcourse) so they stop moving out of reflex? or why exactly does this work for you?

Having owned cats, I can't say the exact reason behind the behavior, but I can confirm this is often the result-- especially from the male. And it's definitely not a fear response. They flop over and just lay there with a very similar expression on their face as the cat in this photo. "I hate everything and life is out to get me, why must you persecute me so, human?" Almost like a little kid throwing themselves on the floor in a tantrum.
The harness should never be tight enough to illicit the same reaction a bite would-- they do not pinch the cat in any areas. It could be a similar response, but I'm genuinely not really sure how that would come about. Either way, this tends to be a reaction cats can overcome with more exposure to wearing them. Seems like a great trick in the meantime though

Our cat can't go outside because we have some sweet wildlife where we moved to, but she still isn't used to staying inside so she meows a lot. We put a harness on her and took her outside thinking we could attempt walking her (even though we never trained her as a kitten) and she just... Froze. Didn't move just froze still! It really does work wonders!