I found out by accident that it can be turned on and off.
Try pressing the black toggle button (I'm not sure which direction) in the center of the large ring on the MMI controls. As I recall, one direction turns it on and another direction turns it off.
2016 A6 Premium+, SLine, Florett Silver Met./Black. 3M Crystalline 40%
2013 Q5 2.0t, Cuvee Silver/Black, Pano, 19", MMI/Nav, B&O, Advanced Key, 3M Crystalline (traded).
2014 BMW 3281 X Drive, White Metallic/Tan, Nav, 3M Crystalline etc.