If I understood correctly he means that you can write a simple tool that reads input from the camera, capture the eyes and as soon as you turn away the pc will shutdown.
You mean a very simple algorithm to detect eyes using the OpenCV (https://en.wikipedia.org/wiki/OpenCV) written on a RaspberryPi that has a tiny camera which connects to your desired computer via network (be it WiFi or cable) and sends a shutdown command once it no longer detects them? I am pretty sure you can find on SlackOverflow (pun intended) / RaspberryPi forums desired bits of software and make do in maximum one day.