Using a “biological amplifier” the muscle signals were amplified thousandfold by shifting the major nerves that normally went down the arm and letting them grow into the chest instead. When you think of closing your hand, a chest section will contract and electrodes will pick up those signals to tell the prosthetic arm to move.
The brain exchanges information through neural circuits, which have receptors to sense a stimulus, report this back to the nervous system and produce an appropriate response via motor neurons which lead to movement.
A touch on the chest would actually lead to the sensation of a touch on the patient’s phantom arm, even his missing fingers. Senses of hot, cold, as well as sharpness and dullness were all felt and this provided a way to restore sensation using a prosthetic hand “that feels”.
A small microcomputer sits on the patient’s back connected to the prosthetic which is trained by the patient’s mind to move in specific directions and perform different tasks.
If you are new to bionic prosthetic technologies, this is a great introductory article about all recent approaches.