The update, which should be present under Software Update, enable’s Apple’s Power Nap, which it describes this way:
When your Mac goes to sleep, it still gets things done with Power Nap.3 It periodically updates Mail, Contacts, Calendar, Reminders, Notes, Photo Stream, Find My Mac, and Documents in the Cloud. When your Mac is connected to a power source, it downloads software updates and makes backups with Time Machine. While all that updating is going on, the system sounds are silent and no lights or fans come on, so nothing disturbs you. And when your Mac wakes up, it’s good to go.
Now that the firmware update is out, the feature has appeared in the Energy Saver control pane in System Preferences. If you’re using a Retina MacBook Pro, hit up Software Update under your Apple menu to see if it’s available for you.