I don't think that pressure points have anything to do with ki... Pressure points are either nerve plexuses or certain structures (tendons / veins) with receptors that will elicit a reflex when stimulated.
For instance, almost all arts utilize a strike to the plexus solaris (it's really called the plexus coeliacus). This is a real 'knot' of nerves, and when struck, it will cause the diaphragm to relax.
An example of a 'reflex' used in martial arts, is the achilles tendon reflex. There are little receptors in the achilles tendon that give feedback to the brain about one's posture. If you're learning too much forwards / backwards, the brain will automatically correct it (hence it's a reflex). If you use a leg sweep, then your foot will strike against the achilles tendon. The receptors then will initiate the reflex. Usually, you don't really notice the reflex taking place, but a correct leg sweep will stimulate the receptors more than usual and cause a reflex that's too strong - and thus, a leg sweep will usually make someone fall. ( I know that there are different leg sweeps - there are also a few that work on the nervus tibialis - when stimulated aka hit, this nerve will send tiny 'electrical shocks' and cause someone to stumble).
And of course, there are also other receptors that can cause a reflex - like the carotid baroreceptors in the carotid artery. You'll start feeling dizzy if these are stimulated. A common technique which stimulates them, is a knife hand strike to the neck. Also, if you gently press your palms into someone's neck (into the carotid arteries on both sides) and give a slight push, then you'll be able to make most people fall backwards without too much effort.
So... do I doubt the efficacy of pressure points? No, because virtually all martial arts techniques depend on them. Of course, pressure points are usually very 'small' points, you'll need to hit exactly the right spot to get them to work properly - but most martial arts techniques depend on accuracy, don't they? It takes some practise to be able to properly execute a leg sweep - and this isn't because of the fact that the technique is too difficult etc, but it's just because you need to hit the right spot in the right way.
Also, this explains why people don't aways respond the same to the same technique. Some people have quite sensitive nerves and some people don't. I know a guy, which also trains Pencak Silat, who you can hit directly into the plexus solaris and he won't feel anything. My instructor even tried it, to no avail.
Of course, there are also the more obscure pressure points, those related to meridians... My anatomy professor showed us some very interesting research, which states that things like acupunture aren't necessarily connected to taoism etc, but that it's just a more 'simple' (I don't want to offend anyone) explanation for something that can be perfectly explained by science. This is because all nerves are interconnected. This causes things like referred pain - some people who are getting a heart attack, only feel that their left arm hurts.
So, for me, it isn't really hard to believe that it is also possible to affect the function of certain organs by hitting a certain nerve, but I do think that such a skill is very dangerous, especially if you don't know exactly what you are doing... I mean, hitting someone in the stomach is one thing - it's effects can be felt clearly and you know why it hurts - but causing someone's pancreas to shut down or something, sounds more like murder to me...
Well, I hope this contributed to the discussion
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