There's not much about it online. They do mention it in the docs, they call this "prefix compression" [0], so you can search for that. This article describes it [1], although it is somewhat high level.
They only use this for indexes. It works really well for internal mongo ids (they all start with timestamps if I remember correctly). It also works well for compound indexes. Pro tip: for compound indexes always go from low-cardinality attribute to high-cardinality attribute to get the best results from prefix compression (it's also good for speed of queries).
