Modern Warfare 2 is considered to be one of the best FPS games of all time, so its no wonder why you are interested in how-to videos for it! Released for the Xbox360, Playstation 3, and PC, Modern Warfare 2 provides an excellent gaming experience for action lovers. Whether you are new to the game or have completed it on Veteran, there's a good chance you'll find some useful information in some of these how-to videos.
Knock-knock is an achievement attained during one of the hostage rescue / breach levels. To get the achievement you must breach a room, and kill every hostile (on your side of the door) in 1 shot. This is fairly easy to do since breach missions are put in slo-mo for a few seconds.
Check out this video for a guide on how to do it for one of the breach missions.