And it is not in destroying the joint,although it could be. There is a deeper mechanism at work in Kyusho joint attacks, that affects the entire neurological system as it radically changes the opponents physiology... not to mention psychology (when your body does not function normally, your mind looses control as well).
What is evident in the study of the Bubishi, was that the writer of the book and creators of specific Martial Method, knew this. I say evident as it is documented in the Bubishi once you have the keys to open this cryptographic record or document.
Basically the joints are tied in with the reflexes, they are in turn tied into the subconscious brain (80 - 90% of your brain operations) as well as the autonomic nervous system. By comparison the 10 - 20% of our brain is the conscious which involves the somatic nerve system, those responsible for muscle actions we control. Typical Martial Arts act from and attack the somatic aspects of the opponent, but that is only 10 - 20% of them and a good reason a body can take a beating and continue to fight. But by affecting the joints properly, you are tapping into 80 - 90% of the human... far more devastating and injurious to the opponent.
This is just a small bit of the full picture and potential, we have greater explanation and detail for our Platinum Subscribers as always.