Request Tutorial on "Detail sprites"

I could explain how to do it all in TF2 in a lengthy picture-filled cool post, but I just use Tophattwaffle's tutorial on the subject: https://www.youtube.com/watch?v=mkZuSVgQFFo

there are a couple detail sprite related map properties you need to change. edit->map properties or whatever file menu tab it's under, then the properties you need to change are at the bottom and by default have "2fort" in them. there are some lists of what textures need what properties, but I don't have them handy.
they automatically get added when you compile the map depending on the textures. there are alternate versions of the textures that make them that have fewer or no detail sprites, if you want to control where there's grass.

How do detail sprites even work? Are they just little models randomly scaled and oriented by vbsp or something? I feel like that would be extremely expensive if that were the case, so I assume I'm wrong.

They can be models but often they are just 2 tri sprites. They have a small hardcoded draw distance and a map has a limit on how many it has, so it ends up being fairly cheap.