A couple of articles; includes this paragraph:

"Battery Conservation. Of course, when a cell phone is constantly listening for a keyword, or waiting for you to shake your wrist, it must burn some energy. When figuring out how to minimize this, the Moto X team drew on conservation techniques the company used on its obscure wrist-based fitness product, the Moto ACTV. They would create a unique architecture for the phone, with eight processing cores. (Thus Google calls this “the X8 Mobile Computing System.”) One of those cores is a super-low-power component that does nothing but listen for the words “Google Now,” enabling the others to slumber away. (If it were not for this approach, says Arshad, such a feature would require three batteries to get through a day.) Another low-power-consumption core monitors sensors—like the accelerometer that signals that you want to take a picture."

ORY