This is an example map that demonstrates a useful hack brought in by Vluzacn's HL Tools.
By applying the
texture from zhlt.wad, which should come with your VHLT download, to one or more of the faces of a brush you'll make all normally textured faces to be mirrored on the inside of the brush, much like with water surfaces.
It has many applications: easens creating foliage and other UnrealEd-like volumetric brushes, allows for prettier light cones, allows to use just one brush entity to create a flat polygon visible from both sides.
More information can be found on the wiki page
: discovery of the feature;Trempler
: sharing it to me;Vluzacn
: has created the tools that still help mappers a lot to this day.