It's called the Magneto-Track system, and it uses low-frequency magnetic fields to follow a tracker embedded in the game ball. The result? A real-time measurement of the ball's position and orientation on the field.
The system has the potential of allowing referees to track the ball in all conditions, even when its visually obscured by a mass of players. Still, it isn't perfect: while tests have successfully tracked a moving football's position across the field, it still has a one foot margin of error, making it useless for precise calls.
Although the team is trying to improve accuracy, the technology could still be useful in its current state -- giving TV viewers a better way to follow the ball during low visibility games, for instance. Want to see it in action? Check out the video below.