Have you ever wanted to add an invisible keyword to a document? Say, you wrote something about *Dog videos on Youtube" and the text does not contain the word “cat”, but you want the document to show up when you search for “cat”.
Well, you could tag the document with cat, but say you use your tags parsley and don’t want to create a tag for this one-time occasion, what can you do? Well, if your document is written in Markdown you are covered:(*)
Begin your document with the Multimarkdown Metadata section! Just have in yor Markdown document a first line
Keywords: cat
followed by a blank line. Below this enter your normal (Multi-)Markdown text. This document will be found when you search for “cat” and at the same time the view rendering of the document does not use the word “cat” at all - it’s all about dogs on Youtube! 
(*) A similar technique can work for other document types like HTML as well.