A command interneuron is a single
nerve which, when stimulated, will coordinate a complex set of movements in the organism.
For example, a crayfish, when threatened, triggers a command interneuron which makes it arch its back and raise its claws up defensively in an open, threatening posture. The Tritonia nudibranch, when accosted by a starfish, its main predator, will trigger a command interneuron which initiates an automatic swimming response, allowing it to flee.
By hard wiring a specific behavior into the nervous system, the animal improves the speed and efficiency of certain vital actions, increasing its chances of surviving.