I first encountered them in English-language books about lesbian sex (as a teenager pre-internet) and hence I long knew that they were these latex squares, but I just couldn't imagine what they could possibly be used for in dental practice, but never wondered enough to tediously research dental stuff. Then post-internet I eventually looked up pictures, but I still can't quite envision how it this all works in practice.

I suspect they are just not often used in Germany, because I'd never heard of them from others either (in the context of dentists), even though the German wikipedia page claims they are supposed to be in the best practice guidelines here too for certain things.