The $displacementmap command is an alternative to $bumpmap that uses a heightmap to give the illusion of depth.
It's the most realistic of all the texture shadowing techniques, but consequently is also the most expensive.
It requires a model compiled with $subd. For best results use a relatively high tessellation level.
To do: Find when it was added into the engine, the earliest example appears to be in Alien Swarm although it works in SFM