Hello Lioncour, the ‘IR Channel’ mentioned is a groove/frame where IR LED’s are mounted so that IR light is shined into the side of an acrylic panel. This light is trapped inside by internal reflection. When a finger touches the acrylic surface this light is frustrated causing the light to scatter downwards where it is picked up by an IR camera.
There is a picture of the technology below from Tim Roth’s blog, and a picture of Falcon4ever’s LED frame showing the mounted IR LED’s.
Image from Tim Roth’s blog
Information directly from the NuiGroup Wiki
Image from Falcon4ever’s blog
Hope this helps!