There are times when you want to subdue an opponent rather than just kill them. Normally, it is assumed that a monster will die when it drops to zero hit points. If the intent is to subdue an opponent rather than kill them, the intent to subdue is announced. Combat continues until the creature goes to zero hit points. At this point, it is unconscious, but not dead. If the attacker acts quickly (before death saves run out), the creature can be bound or otherwise restrained for questioning (etc).