When I was in Japan twenty years ago, I noticed that high school students would sometimes study English from special books with black and red printing inside. The book would come with a red-colored transparent plastic film, a little smaller than the size of a page, which would be used to obscure words that the learner was trying to guess. Words printed in red would be invisible under the transparency.
Here are some photos to give a better idea how it works:
I'm wondering about the popularity of this learning method, since I've never encountered it outside of Japan. Was it ever used in America? Is it still used in Japan? Is there something wrong with the method? Was it just a fad, or was it another instance of Japan being ridiculously advanced education-wise?
Does it even have a name?