The NSA-B-GONE sits between your X220's motherboard and the 30-pin flat flex connector that provides power and data to the webcam, microphones, ThinkLight, and screen-mounted indicator lights. The onboard ATtiny88 microcontroller monitors the switched ThinkLight wire, and when it sees the user flash the light twice in quick succession it disables a pair of tiny USB data switches, physically cutting off access to the webcam and microphones.
A word of caution: this board, along with the other custom PCBs I've designed and shared that fit in an X220, could destroy your laptop if you don't know what you're doing. I installed, programmed, and debugged this device while my computer was powered up, but that doesn't make such behavior a good idea. If you make an NSA-B-GONE, be sure to carefully inspect your solder joints, program it outside the computer, and only install it when the computer is off, disconnected from AC power, and missing its battery.
For more information, build instructions, source files (firmware and hardware), etc, head to the project repository: https://github.com/zakqwy/NSA-B-GONE.