Most Martial Arts work (or focus) on the surface of the body, which with massive power hurts a lot and can break what is hit.
But there is a reason why they need to develop this massive power (which does diminish with age) to inflict the damage and that is two fold; first is that they target the shell or surface of the body as they attempt to damage skin, muscle and bone.
As they attempt to damage the bone they come up against a protection inherent in nature and animal structure, that is the curve of the bone or skeletal structure. The skeletal system is designed to protect the more vulnerable inner structures, (such as blood vessels, nerves, tendons, ligaments, and organs) and in part why it is shaped as it is, with curved and mobile construct.
The curves dissipate oncoming pressure away from the impact area just as a suspension bridge transfers weight to the end support structures... and yielding greater strength or weight bearing capability.
The Skull can take considerable impact due to the bone thickness, but mostly due to the curve. When a shock impacts the curve of bone, the kinetic energy vibrates along the surface as opposed to entering directly., this protects the inner structures such as brain, nerve and blood vessels. If the impact connects with the jaw, the curve again is at play, but additionally there is the mobility and flexibility of the hinges to further absorb the kinetic force. There is also the extreme flexibility and vast mobility of the neck structure to allow the force to be somewhat negated by the movement away from the source of impact.
When we impact the upper torso we run into the "arch disambiguation" issue once again. Looking at the curvature of the ribs, collarbone, spinal column, we again see the arch at play, add to that the mobility Bending and rotation, along with flexible connecting hinges we see that incoming energy is to some degree, diverted away from impact location.This negates the full force so to overcome this one would need substantially more force to cause the desired result or damage.
The Hips and Legs
Again we see the hips as a labyrinth of curves and although not flexible in and of itself, it is suspended per say in between flexible and mobile structures as well as hinged connections. The Thighs are frontally curved and can take substantial impact, the rear is protected more by mass of muscle for shock absorption. The bones are also round to help with the dispersion or arch disambiguation, as is the structure of the foot.
To enable greater affect with far less energy expenditure, we can avoid the direct impact on the skeletal structure. Targeting for us occurs in the small spaces between these hardened and arched skeletal bones for deeper penetration, higher efficiency with far less power and far greater affect.
There is no mystery to Kyusho folks, just plain old science.