Ask HN: How does the FlightRadar's mobile app work?

Flightradar[0] is a pretty cool service which has a live map of airline traffic throughout the world. The mobile app has a feature of being able to identify the exact flight name by just pointing your phone at the sky. When you point to the sky, you are asked to calibrate by rotating the device multiple times while keeping the focus on the plane [1]

While I understand how FlightRadar works, with just my GPS coordinates how does the app correctly identify the plane that I'm pointing to? Basically, how does it get the direction I'm pointing towards to narrow down the list of planes that are flying within K miles of a given lat, long coordinate?

[0] - http://www.flightradar24.com/

[1] - http://youtu.be/IqFx77WeYL8?t=4m27s