It replaces various textures to make them look like an even higher picmip. These picmips cannot be used without sv_cheats on, so this mod may be one of the only ways to use have lower quality textures unless you figure out one of the more risky/complicated methods of doing so.
Whether or not this would increase performance is something I can confirm to be true. I don't know by how much though.
And that's about it. Enjoy.