Quote from Bic-B@llif someone told me about this stuff a month ago i would have called them for bullshit quite strongly, i would have never believed that the half life engine was able to dynamically let you see into other areas of the map, let alone see players. id say its quite surprising that its part of the engine, maybe they added a whole lot of code to the engine that will let developers do even more crazy stuff with mods. Who cares if its a leap ahead or not in games.
I don't think its that difficult to implent actually: