The nerves grow into the chest muscles, so when the patient thinks “close hand,” a portion of his chest muscle contracts and electrodes that detect this muscle activity tell the computerized arm when to close the hand. Thus, the patient thinks “close hand” and his artificial hand closes.
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/blog/show/first-woman-to-receive-bionic-arm