Yonkajo or yonkyo (as referred to in some Aikido styles) is a
painful wrist pin technique.

It involves a sharp, sudden pressure upon the nerve centers of uke's forearm, usually on the inside near the wrist (although it can also be applied on the outside of the arm). This technique assumes that nage already has a hold on uke's forearm, after having successfully

evaded his opening attack. In most of the applications of yonkajo, nage will extend his arm an instant after having established a hold on uke's arm and cut down with the inner edge of his hand across the tendons, pressing deeply into the complex of nerves located two or three inches above the wrist. The little finger and the root of the index finger play an important role in applying the pressure.