Sankyo is the third technique in Aikido (san –three, kyo –teaching). It is a common wristlock (easily recognized by anyone who has practiced judo or ju jiustu) which involves forced supination or pronation of the wrist. Sankyo is typically applied by holding and twisting the hand. For added suggestion, one can grab the small or pinky finger with the lower hand.
Sankyo is effective for the reason that the wrist joint does not mechanically allow for a full degree of circular rotation -things like bones and tendons tend to get in the way. Accordingly, force is transferred to the forearm causing radioulnar rotation, eventually resulting in a joint lock on the radioulnar joint. Once the radius and ulna have been brought to their extreme positions, further twisting motion will put severe torque on the wrist.
Sankyo is often accompanied by an opponent instinctively throwing him or herself to escape or alleviate the surprisingly painful lock. Sankyo is typically applied for control and should not cause long-term damage. As in the case of Shihonage, the advanced Aikidoka will refine Sankyo to generating torque through your hips and not your arms.