In the different kinds of Aikido locks, they are used on the bones, joints, ligament and tendon. Among all the different locks, Yonkyo is the most "special" kind of lock. In Yonkyo, the inner edge of the hand (between the thumb and the index finger) is applied to uke's upper wrist. The force is applied to the side of the part above the wrist. 
Using the anatomical study of science, the force is applied to the tendon that hold the muscles of the hand and the upper arm. Locating near to these tendon were the ulnar nerve and median nerve. The Ulnar Nerve is one large unprotected nerve. With a certain force applied to the ulnar nerve, it will send sensory impulses to the brain for the sensation of pain. Therefore Yonkyo is known as a nerve pinch. The pain is more of a sharp and electric feeling of pain. But Yonkyo is not a lock that can be applied to every person, firstly the finger coordination must be correct. Sometimes a person arm is protected by a layer of fats or the different position of the nerve. How to do a correct Yonkyo???
- The angle of the arm must be 90 degree
- Press downwards
- Ensure that the uke's elbow is below the level of your fingers that applied the lock
- Ensure your arms are straight
- Do not move the uke's arm sideward
- Do not hesitate on pushing down the uke's arm
