The Moto X has a great feature which allows Google Now to be triggered with a simple ‘Ok Google’ voice command, which is made possible due the device always listening for that particular phrase to be announced.
Other devices such as the Galaxy S5 can be trigged using the ‘Ok Galaxy’ voice command, but only when S Voice is actually open. It seems, however, that this background listening is not constrained to just the Moto X and is infact embedded already in the Nexus 5. A recent video shows a concept which unlocks this native compataiblty in the Nexus 5, allowing Google Now to be triggered with the simple phrase hands-free.
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The concept is part of the custom OmniROM and it seems to work pretty well. Hopefully the developer can build the feature into a finalised package for deployment to the general public in order to bring true hands-free operation to the Nexus 5.
Update – It seems that the video has already been taken offline.