Sensing Computer Data

We tested related methods, so this research recalled some of the ideas.  How do you extract data from unconnected computers?  By sensing other things computers emit.  In Wired:

IN THE PAST two years a group of researchers in Israel has become highly adept at stealing data from air-gapped computers—those machines prized by hackers that, for security reasons, are never connected to the internet or connected to other machines that are connected to the internet, making it difficult to extract data from them. .... "