Kyusho involves 3 main layers of training and anatomical targets.
Where only the first main layer is needed to be highly proficient and effective in protection, the avid practitioner can rest assured that there is far more to learn that can increase your potential infinitely more. (please note that several sub-levels exist for each main level).
Focusing on the first level we find the access to the peripheral nerves and tie into the central nervous system of the opponent, this is the most accessible and easily trained level as it is the safest to practice realistically. The nerves are very resistant to damage and cause such immediate response that they remove the individual from more impending or damaging input. This is natures design as it has it's own layers of protection for the human of sense, fight or flight response, and reflex... by tapping into this we can immediately disable the opponent and their ability to continue their assault. Only after this layer is fully understood and practiced into realistic skill, is the practitioner able to advance into the next layer.
The second layer is the blood system, but with this comes a need for much higher skill, understanding and caution. The blood system is far more susceptible to attack and why it is protected mostly by the nerve system. The nerve system in part protects the blood system with reflex and other responses or reactions, this is why far more skill is needed to access or inflict on this physiological system. The term (so loosely used in modern times), was originally named Dim Mak, or death touch, this is indicative of it's potential on the human body. As it is a more vulnerable target realistic training is not possible without potential risk (as are the nerves). So attaining a higher level of skill with it becomes very difficult although not impossible.... make sure you are fully aware of all implications to you or your partner before attempting and really listen to the instructor for signs that they are actually competent in this prior to actual practice of it.
As with the first neurological level, there are first aid methods you must know prior to training (this goes far beyond the lifting of the legs involved in a vaso vagal faint is. The practitioner must also realize their limitations on this level of first aid as it is not as quickly or cleanly remedied as is an attack of the nerve functions of the body... in short do not practice this unless you are absolutely sure of what you are able to fix and never step beyond these boundaries. As example stopping the blood flow into the brain (via blood chokes at the carotid sinus or other targets), can have severe and permanent damaging affects.
The third level is not public domain as all the above cautions, difficulties and potential damaging affects escalate exponentially and must be reserved for only the few that prove they can handle it properly).