The glitch requires two players. Player one creates a COD2 system link game, selecting the player limit, game type (including kill count), level, and time limit. Now, player two, while playing a different game, invites player one to join. Player one must initially accept the invitation, but when prompted to log out of COD2, player one must refuse. These actions will trigger the glitch, which designates player one as the host of an XBL match using the parameters chosen for the system link match.
If you wanna get in on the action, DUKEforLIFE, one of the first to discover the glitch, has started a thread targeting prime hosts. But let's hope Activision & Infinity Ward get a clue and issue a massive update, so we won't have to rely on glitches to play a decent round of COD2.