Ok so I understand that an observation mirror becomes transparent if one side is light but what is not clear from the article is that any bright part apparently becomes transparent.

Because I assume the author has a black screen, and white text, causing the white text to shine through the mirror while anything around the text is still black and let's the mirror act as a normal mirror.

Also it would be nice to see which observation mirror he used, since he's Dutch I could probably order the same one to Sweden. Ordering from the US is often a hassle.