Google has introduced a new Power Saving mode in Android L to help users extract maximum juice from their battery. Enabling this mode will automatically disable the animations, clock down the CPU speed and turn off mobile data to preserve battery life.
However, it looks like, Google is also working on a Low Power mode in Android L, which it has buried deep inside the Development menu.
Right now, it looks like the feature is not fully developed since enabling or disabling it had no visual or performance effect on my Nexus 5. It is also different from the Battery Saver mode located under Settings -> Battery, since toggling the latter had no effect on the former and vice versa. It is possible that Google is working on adding an Extreme or Ultra Power saving mode in stock Android, just like HTC and Samsung have introduced in their latest flagships for 2014.
To activate this hidden Low Power mode on your Nexus 5 running the Developer Preview of Android L, head over to Settings -> About Phone and tap on the Build Number seven times. Then, head back to Settings -> Developer Options and scroll down to the Monitoring category and enable the Low Power mode from there.