The feature is effetively a levels app. After calibrating the Compass by rolling a little ball around, just flick right to a second screen to access it.
Dave threw out the challenge to get a screenshot of the compass at exactly 0 degrees when it turns green. Well, in a few minutes of boredom yesterday I decided to give it a go.
It took multiple tries to get it at exactly zero with the full green tint. The moment you press the buttons to take the screenshot you naturally move, so it's not easy to get it, but after a few tries I managed to get it:
Yes, it's a bit silly to try and catch the green 0 degrees. But the actual feature is useful if you ever need to ensure something isn't at a slant.
To close off, here are a few tries: