Now, look at the knot itself, the chord is tightly wound, so, impact is in very small areas, several at once, which concentrates the energy, and is multiplied by the leverage of the lanyard attached... I have gotten the thing stuck in a solid piece of mahogany.
It will easily decimate occipital bones. There are many small, light bones and processes (like tunnels through bone) near the eye, the monkey fist can cause severe and irreparable damage very quickly.
This is not an offensive weapon, is for defense only. Last ditch type thing. Not a perfect weapon, but very effective if employed correctly.