Looks like some posts I put up vanished in the RAID crash. So fwiw I'll try to put some different posts together in to one and add a few thoughts that occurred to me afterwards. All apologies if it seems random, but...
Many moons ago I enjoyed some time in Judo. I recall a kindly, small, spindly old fella demonstrating hadaka jime on me. I still *vividly* remember my initial reaction which was like my head was about to pop off just like a pimple (sorry, but it is about the best explanation of the feeling). I agree that there are times when it feels like the pressure in your head is going up. I don't know if it *really* is, but it is a distinctive and unmistakable feeling. And it seemed like only a few blinks of an eye before I was fuzzed then gone.
There is a distinctive "Oh CRAP!" feeling that happens when it is applied. And frankly I've seen it happen a few times for the first time for some people and their reaction is always the same -- big eyes, jolting, etc. I think there is some sort of deep reptilian part of the brain that reacts to having the flow cut that is simply normal and natural. And if you're not experienced in what it feels like the reactions are fairly predictable as Mr. Clark has already pointed out.
Not that long ago I was chatting about martial arts with a friend of mine who is *highly* experienced in knife fighting styles. Well, we got talking about things like hadaka jime (rear naked choke in my understanding). I said that properly administered it can be a very difficult thing to deal with. His response was "well, not if I have a knife in my hands". So since we were talking and we had a training knife we decided to see what would happen. My only request was for him to allow me to get in to position cleanly first because I simply didn't want to run the risk of damaging his neck or throat. So I got in to place with no pressure and said "okay, remember to tap out if you need to, but here I go". So I tightened the jime and the first thing I feel is his tightening up (normal - they sort of "jolt" especially the first time they've ever felt it) then I heard his knife hit the ground. I let go immediately thinking "Oh, crap, I must have hurt him." Well, no, I hadn't. He had never had one applied. When it clicked in he simply dropped his weapon and started to flail about. That's when I let go thinking I'd hurt him. So an *experienced* guy who on any day can filet me from here to Sunday with decades of training with that specific weapon *dropped it*.
Of course this is just one case, an anecdote, but also consistent with my experience of how people react to a properly applied hadaka jime. There's not much escape there and given the shock and time constaints *if done correctly* it is a devastating and rather decisive technique. In my experience at least.
Of course he let me get in to position cleanly. But this was to test the idea. So there are issues there as well.
Another observation is that the instances I've heard of regarding injuries are mostly during untrained idiots doing what they've seen on the tube (either tv or youtube) and hurting each other. Other instances are the LEO involved incidents where they were using more of an arm-bar across the windpipe to crush everything. Good for pain compliance, I suppose, but significantly more dangerous especially if the person receiving is flailing or isn't feeling pain for whatever reason. I used Hadaka jime as my example of the technique because properly applied it won't even tend to leave a bruise let alone any serious damage. Of *course* it involved shutting down the brain due to cutting off oxygen flow to the brain. That by definition is of course more dangerous, but I've not heard of anyone ever being injured in training doing this sort of thing "correctly" (i.e., well trained). Now I've seen and received injuries doing any variety of other things in Aikido that hobble me today. But no ill effects that I know of from being choked out a few times. Hmmm, maybe that pillow drooling... Have to think about that...
Anyway, my experience is that it is a devastatingly effective technique done correctly. I do not practice (nor would I ever think of teaching) any windpipe crushing techniques. Too dangerous IMHO. I have, however, taught classes in Aikido about chokes (kubishime) and when I have the right students in class I'll demonstrate what it feels like to have a hadaka jime applied. I have never let one go longer than just a brief second -- that has been overwhelmingly convincing to each person thus far that it is effective. I do this to remind them why it is so important to avoid getting caught in one. There are things you do to make it difficult to get something like a hadaka-jime. And I want them to understand the difference between someone choking painfully on the windpipe is a different event from someone cutting off flow. And frankly many "chokes" applied by amateurs are either extremely dangerous or completely useless as they don't know what they're trying to accomplish. So the teaching is about making sure students understand what's going on and how it feels.
Hopefully I gathered together all my old points.