A player that takes enough damage is incapacitated, and can no longer take damage. That player can still communicate over Voice or Text chat until they are secured, and can be revived by friendly players.
Takedowns and Melee
An enemy player can be neutralized from behind by performing a Take-down, Left [Ctrl] key by default. Take-downs do not alert nearby enemies with sound, though the victim may still use Voice chat to alert nearby teammates.
Reviving Friendly Players

Securing Enemy Players
