After 31 hours on battery I finally got down to 15% and I was asked if I wanted to turn on battery saver. The pop-up explained that Battery Saver will automatically disable background data
when the battery is below 15%. I guess the idea is that you might stop certain apps from syncing, and might stop receiving emails (unless they are a special exception to background data?), but you'll still be able to receive phone calls and texts.
Here's what the prompt looked like in the notification bar and then when I clicked on the notification:
I don't think manually turning this switch on means that it will ALWAYS disable background data (you can do that elsewhere), so the quote from the droid ultra manual is kind of confusing.