I gathered some commands that will use the SourceEngineFog to make only the area visible in which the damagereduction will take place. Behind the black border damage stays the same before the update.

To enable this start a server an use this commands
sv_cheats 1; fog_enable 1; fog_override 1; fog_color 0 0 0; fog_end 512

Picture -> http://www.pic-upload.de/view-1549364/DemoRange.jpg.html

To Reset to old values use this commands
fog_enable 0; fog_override 0; fog_color -1 -1 -1; fog_end -1; sv_cheats 0

Quantum

ps
you can use the “fog_start” and “fog_end” command to shift the area in which the fog will be. The default value of this commands (i think) should be “-1” if you want to reset later.