Instructions
Boot up your gaming system and wait for "Call of Duty 4: Modern Warfare" to load up. Select either to start a new game or continue with a current game.
Move your character up close to an enemy. This is often dangerous to your character's health and should only be done as a last resort.
Push down quickly on the right thumb stick in both the PS3 and XBox 360 version (the thumbstick is also known as the R3 button on the PS3 controller) or the "V" in the PC version. This activates the knife, which is stabbed in front of you. Do not hold down on the stick, as this activates the aim feature for your main weapon. Quickly tapping and releasing the stuck performs the action. A direct hit kills your enemy instantly.
Repeat the process as often as you choose to use your knife.