It is simply an extension of the phong shader. They Highlight the model. Of course I'm expecting people to say something along the lines of "It gives an unfair advantage!!!" Well guess what. It doesn't. The maps are generally well lit enough to recognize a player from a very long distance, plus you can't really make out the rim lighting at that distance anyways.
Settings/Alpha Mask created by - Jake (FrostbiteX)
To install place in...
\Steam\SteamApps\\Day of Defeat Source Beta\dodsbeta\materials\
If there is no materials folder than simply create one!