They combine a thermal camera with a normal camera, and use augmented reality with thermal heat tracking, to turn surfaces into a touchscreen. Anything you can you can touch meaningfully, I guess....
See the company blog post promoting the technology:
"Thermal Touch is a radical new approach to wearable headset graphical user interfaces (GUIs). It utilizes infrared cameras to register and track minute thermal imprints left by the heat signature of a finger. Touch your desk – you’re leaving imperceptible (and impermanent) heat maps each time your finger touches the surface. "
Our mobile prototype is based on a tablet PC to which we attached a combined thermal and visible light camera module. The fixture is simply a joist hanger I bought at the next do-it-yourself store. Our proof-of-concept software implementation is based on the Metaio SDK and therefore features the latest tracking capabilities for dealing with both planar and three-dimensional objects. It further provides the functionality to render virtual objects registered with the tracked objects.
We had to extend the Metaio SDK to support capturing images from the thermographic camera and I developed a prototypical touch detection algorithm. All in all it wasn’t really that much work, because most pieces already existed in our SDK. The last thing to do was then creating some exemplary applications to demonstrate the versatile opportunities this technology offers in different use cases.