Google added a battery saver mode in Android 5.0 that disables various features when you need to conserve juice. You could activate it manually or have it flip on at a certain battery level. Android M adds a third option—voice.

The implementation is a little weird right now, but it works just fine. If you tell Google, "enable battery saver mode," the device will do as you command. However, the audio response tells you it's opening the battery saver settings, and the same thing appears as a card. You can just close the voice actions interface, though, seeing as the task is already done. Disabling the battery saver can be done by voice in the same way, with the same wonkiness.

This is a preview, so I assume the interface will respond correctly in the final version. On Android 5.0 this command does just open the settings. It's a handy feature, just a little confusing right now.