From Valve Developer Community
It is used to mark surfaces that shouldn't receive dynamic shadows. Useful to apply to walls and floors where shadows are drawn improperly, giving away the location of enemies.
Bug: This entire entity class is a hack to mask shadows appearing on the wrong side of walls.
Note:This is an internal entity. When the map is compiled by VBSP it is processed and then removed: it does not exist when the map is running.
- Brush faces
- List of brush sides that shouldn't receive dynamic shadows.