When an enemy comes into close proximity, or if your weapon/side-arm is out of ammo, you can try and strike your foe with your knife.
It is usually used as a last resort as there are a few downsides linked to using it -
- The time between slashes can leave you open for attack.
- It takes two slashes to the chest to kill an opponent with heavy armor and full health.
- Missing wastes vital time.
*Note that there are perks that you can aquire to give you a more promising outcome with your knife.Edit
Although the downsides look bad, there are a few upsides to using your knife -
- Stealth - you will not appear on the minimap as no gunshots were used.
- A well timed slash to the back can kill in one go.
- It's possible to silently kill attacking soldiers from behind enemy lines.
It is not a recommended tactic to purposely kill with the knife, when such tier 3 weapons can do the same with great ease, and from a further distance.