From An Inquiry into Application of Gokyo (Aikido’s Fifth Teaching) on Human Anatomy, Perceptual and Motor Skills. Olson, 1996 vol 82.

Using a cadaver/anatomist-observer model, the authors observed that the tissues manipulated by the technique were primarily on the dorsal side of the wrist, proximal to the second metacarpal.

Apparently, “with additional pressure or uncontrolled hyperflexion to the (wrist joint), Nage has the ability to tear the ligaments away from the bone … when this happens, the joint will dislocate”.

The Aikidoist could “break the styloid of the ulna (also involved in the nikyo pain mechanism) or fracture one of the carpal bones.”

Dorsal side of the hand showing Gokyo-affected regions

It is interesting to note a law-enforcement variation called a “gooseneck” which is similar to the gokyo of Aikido; instead of hyperflexing the wrist, they hyperextend it.