I was there. You point the flashlight on your phone at your computers webcam. The software (Javascript / Canvas I think) lets you draw with the light, while you can change colours with your phone. The bonus part was where they used the intensity of the light as a Z-axis, so by moving the phone closer / further away you could build a 3D light painting in the canvas.

Reminds me of this awesome project that a buddy of mine at Google did:

https://www.justareflektor.com/

I think it depends on your notebook's webcam: Webcam recognizes your smart phone's flash (flash = cursor location) and allows you to draw on a virtual surface...