"SecondLight works by replacing the standard glass table-top of the Surface with an LCD panel. This can switch rapidly between frosted - as with the normal Surface - or transparent, doing so at a rate imperceivable to the human eye. The projector underneath this LCD panel is synchronized to display either the standard image, when the LCD is frosted, or the secondary image, when the LCD is transparent.
In this way, both images are produced sequentially but at such a speed that the eye believes them to be simultaneous. Alternative uses suggested included showing more detailed mapping data on the handheld display, when held over a satellite image, and plotting out star constellations when held over an image of the night sky. Microsoft have not given any indication of whether this development to Surface will be included in future versions of the table."
Source: Slashgear
