Several things to cover here. While I'm no expert by many means in martial arts, I have belts in judo and tae kwon do and have also studied jujitsu, arnis, and other forms of fighting/self defense, as well as pursued a lot of written material on the subject. I have experienced the power of chi in my studies and whether it is something that transcends simple physics and body control...it's hard to say, but I can't deny that it exists. Perhaps this "energy" they speak of is simply that generated by the nervous system in combination with muscle control and body alignment, personally I like to think it's more than that.
As far as dim mak, or whatever your preferred art calls it, the fastest way to incapacitate or kill something is to stop nerve conduction. Blood loss or constriction is fast but not instantaneous, I have been choked or choked others and it takes at least 5 sec of good
pressure and that is on a willing opponent. This would only lead to unconsciousness, death would require longer times. So the fastest way via strike would be to attack nerves, disrupting these can practically incapacitate limbs (I've felt it) and I would imagine could potentially lead to death if applied properly.
I have read about techniques like iron palm and other force focusing methods where one can explode bricks and other items though chi force. Supposedly these can be focused at specific distances from the palm, as mentioned by HappMatt. I have no direct experience with this so I can't say how it is done or if it meshes well with combat techniques. It seemed to require a lot of focus and meditation so may not be as powerful under fast combat situations.
Overall, such practices may simply be a combination of physics, training and focus that were more easily accepted as some etherial force rather than a pragmatic description of anatomy and physiology. Regardless of this, they are certainly impressive and again, I would prefer to think, something greater than simple biophysical actions/reactions.