[Philip Lelyveld comment: the video appears to be narrated by Steven Hawking's more articulate cousin.]
BMW is one of the companies that have been the most aggressive in its pursuit of developing this technology, so much so that it created a short video to briefly explain the machinations of its Head-Up Display.
The basic gist of the technology is properly explained in the video by no less than Dr. Rolf Gengenbach, development manager of the Head Up Display Development. If you didn’t understand what the tech does, Dr. Gengenbach properly explains it, both from a technical standpoint and the user experience of having one in the car.
See the full story here: http://www.topspeed.com/cars/car-news/bmw-explains-its-head-up-display-technology-video-ar172481.html
Direct video link - http://www.topspeed.com/cars/car-news/bmw-explains-its-head-up-display-technology-video-ar172481.html