It’s basically a tag-team of spy and sleeper agent. The “spy” listens to electrical chatter in a brain region that processes pain—along with dozens of other tasks—and decodes it in real time. Once it detects an electrical signal that suggests “pain found,” it sends the information to the “sleeper agent,” a computer chip implanted in the front part of the brain. The chip then automatically triggers a light beam to stimulate the region, activating neurons that can override pain signals.
Sources: https://singularityhub.com/2021/06/29/a-new-brain-implant-automatically-detects-and-kills-pain-in-real-time/
https://www.nature.com/articles/s41551-021-00736-7
https://www.eurekalert.org/news-releases/789517