func_viscluster
From Valve Developer Community
func_viscluster is an internal brush entity available in all Orange Box games. It allows you to manually tell VVIS that the leaves inside its volume can all see each other. This is useful in large open areas where visibility calculations would otherwise take a very, very long time to complete. It should be textured with tools\toolstrigger.
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. Note:This entity does not change the location of or number of leaves in a map. See Visibility optimization for how to do this.
Warning: The brush should not cross the surface of a water material. If it really does need to be used over water, bring the brush level with the surface of the water on one side, and then create a second, separate func_viscluster on the other side. Never allow a func_viscluster brush to cross a water plane, or very strange things happen.
Bug: A viscluster will appear to be merging leaves inside its volume together in Hammer's visleaf preview (Map > Load Portal File). This is not actually happening!