The way I understand it, is that it's a streaming program that plays all in-game activity 90 seconds after it's happened. That way you can watch a game, but cannot inform others of where another player is for instance. There's some specific method to use it, it's something like connecting to the server IP but using a different port. So if a server had SourceTV enabled and ended in a 27015 extension, the SourceTV would be 27016 or whatever different IP.
That's my understanding of it anyhow.