Example: on the lock screen, tap and drag the camera icon up a little, then let go; it drops and bounces a little. Now, drag it up much farther, then swipe it down like you want to bounce it—which is exactly what it does. Same with Notification Center: swipe down from the top just a little to trigger it, then let go—it has a slight bounce once it lands at the bottom of your display. Now swipe down a little to trigger it again, then swipe down as if your life depended on it, and Notification Center bounces accordingly.