system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how a amputee controls a bionic arm.
gpt
There’s electronics that picks up the signals sent to the remainder of the arm and translates this to movement. 
When you tell a muscle to contract you send a very weak electric signal. The bionic arm will have EMG sensors that pick up the potential difference between a reference point and the signal source (usually the head of a muscle). Data from multiple sources is used to detect the desired movement.