Correct me if I am wrong but doesn't the snapdragon 800 have some sort of low power listening chip, like in Motorola's x8 system? This would explain why this is exclusive to the nexus 5 because in the launcher the phone is always listening for the "OK Google" hotword.
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We're not sure if the chip can do it or not like the X8, but the Nexus does not. You have to be on the home screen and have the screen on in order for it to work on the Nexus. The Moto X can do it when the screen is off or from within any app, screen, etc.