Many people see the difference in External and Internal Martial Arts as being hard and fast as compared to soft and slow in physical action that makes the distinction.
However it depends more on the mind-set and method of the practitioner as they work if their style or art is truly internal or external.
Kyusho is an internal style as the mind-set or method is based on reaching in to attack or restore the internal structures and functionality of the body. To be able to affect the functionality of the opponent and self, as opposed methods or physical actions to condition or affect only the external body, one must become an internal Martial Artist.
To clarify this further, a Tai Chi Player (generally considered an internal Art) that only performs the form or push hands would relate more to external features. Whereas a Shotokan practitioner (typically considered external), who utilizes Kyusho would be actually using an internal method. This is not style specific measure as opposed to practitioner specific measure.
Kyusho has transformed our old perceptions and gave given us deeper understanding and possibility, it forever changes your personal style for exponential growth and depth once adopted, no matter the style.
The Art/s one practices can no longer be labelled or typify that individual, but rather we must look at the individual to classify their style... what are they doing with that style or art.
As the internal arts were typified as more advanced, we once wondered why this old precept originated, but now fully understanding and adopting Kyusho into our art, we fully understand and can advance ourselves to that internal method.
At the core of any human venture comes the anatomy and the control of all physical and physiological function. But it is not just knowing about it, it is in being able to control it and make it not only physically possible for ourselves, but able to also work upon others.
This is not just martial application, it also pertains to the energetic and functional processes of others. The internal arts can only ever be partially realized unless it can be transferred within another. So the simple manifestation of the Tai Chi player developing their own internal energy as example is only partial in it's manifestation and understanding. (please include all previously labelled "Internal" styles of Qigong and Soft style "Martial" arts).
It is only in the full integration of self and those influenced that any art can become complete.
This is not to say that these "Internal" styles are incomplete, that is only held in the mind of the practitioner as to the merits. But just because a style is slow, or Qigong focused, does not mean they are fully or truly internal. We must always strive for the truth within ourselves, not deception to believe incomplete or labels are concrete.