It’s refreshing the screen 30 times per second (30hz) when it’s static and you’re not touching it. That’s half of the standard 60hz that you’ll see on other iPads. Every 4 milliseconds, it polls for screen or touch activity. If it sees any, it transitions back to 60hz and animates fluidly. It’s a clever trick, and it pays off.
I wish all of my devices could do that. Usually I'm only reading or modifying text, so I really don't need the 60hz. Once something animates it can switch back and I could have the best of both worlds – minimize power consumption and smooth renderings.