This is the highly-effective method of silent killing as taught to members of the SAS. Essentially, you sneak up behind the enemy, and whip out your trusty dagger. Operatives will typically hold the knife in their right hand as they are usually right-handed. If they are left-handed they will use their left hand. (It really does not matter which side they attack as either will result in a lethal blow.)
You cup your weaponless hand over the enemy's mouth, and pull their head to the side of the body of the hand you are using. This exposes their neck.
You then bring the dagger down in a swift thrusting motion and plunge it to the hilt into their neck. Depending on which hand you are holding the dagger in, this will puncture either the jugular vein or the carotid artery. Then you keep your hand cupped over their mouth to prevent them from screaming. They will die within 60 seconds.