In a self defense situation, you might need a knife hand strike, but how hard/soft do you have to strike someone's throat with it to just knock them out without killing? Does it have to be relatively soft or hard?
I have been thinking about how best to answer this for a few days. These particular techniques that involve hitting the neck are problematic for me. I know the theory of how they work. I have practised the move hundreds of times. I can find the spot with my little finger on my own or a training partner's neck. But I have never actually had the opportunity to actually hit that spot in training. Nor have I ever been in a self defence situation that requires it.
For this reason I can only tell you what I have been taught.
There is a part of the neck called the carotid artery (or two, you have one on the left and one on the right). It runs up from the heart on either side of the body and divides at the bottom of the neck into an internal branch and an external branch.
The external branch has sensitive nerve called the vegus nerve. This is called 'Matsukaze' in Japanese or Stomach 9 in Chinese traditional medicine.
I am reluctant to talk about this in terms of acupuncture or acupressure or other Chinese Traditional ideas. I don't feel it's helpful for two reasons:
Firstly chinese traditional medicine does not hold up well in it's own right to clinical trials.
Secondly the number of people (I hesitate to call them martial
artists) that claim to be able to do bizarre things using acupressure points is quite significant. I have even seen videos of people being 'knocked out' from points on the ankle or arm! Obviously this is not possible and there is a lot of psychological
conditioning/brainwashing going on there to make them believe such a thing.
That said there are parts of the body, known as kyusho that are 'tender' and hurt a lot when you hit them, rub them or press on them. Matsukaze is one of the more extreme examples of kyusho.
To stun someone using matsukaze doesn't take much effort but the point has to be exactly right. My instructor's instructor demonstrates it here.
The obvious question here though is 'what happens if you miss?'
As I mentioned before Matsukaze is just one point on the carotid artery. In fact if you strike any part of that artery hard enough you will get much the same effect. (though too hard and you will cause a spinal injury).
So basically you have a light tap/slap in exactly the right spot or a harder punch/knife hand/arm strike a little above or below. I have been taught to use a percussive strike similar to hitting a drum for this. ie. Strike and immediately pull your hand back rather than striking through.
Other Neck Strikes
Worth mentioning here is that the internal carotid artery can also be used. However this is much more dangerous (potentially lethal) and involves poking your finger deep into the neck, next to the windpipe.
The windpipe itself can also be attacked. If this is crushed it won't reopen due to the cilia acting a bit like valcro.
The back of the neck can be attacked in various ways as well... Strikes here are banned in most competitive sports due to the dangers involved.
Please note that other than the above and a few points on the head Kyusho will not generally cause a knockout. At best you can expect a spinal reflex 'flinch' (similar to a Medical Doctor hitting your kneecap with a hammer).
Unless you are trained specifically to capitalise on this kind of movement you are better off avoiding these for self defence... And certainly stay away from any school who's instructor thinks they can stun you by touching your arm or leg.
Other than that you may want to just hit them as hard as you can if you believe your life is truly in danger. courts in most jurisdictions acknowledge a right to self defence in one way or another. You may wish to look up the law where you live or consult a legal professional.