If you didn’t know already, the Nexus 5 actually has a low-power audio processor for “always-on” listening (OK, Google) functionality. However, due to the limited functionality of the feature and its closed-source nature, it only works on the home screen and not when the device is asleep.
However, developer Guillaume Lesniak from the OmniROM team has managed to access the dedicated low-power audio core on the Snapdragon 800 chip to make use of this feature. The functionality is only a proof of concept for now, and although we could see it in later ROMs, it is unlikely because of Qualcomm’s re-distribution licensing issues. Hit the break for a video of the feature.