It prevents a straight forward tracking and forces anyone wishing to track you using a cellphone to throw a lot more resources at you than before. Inside the case it doesn't get any simple tracking info, the only thing it really gets would be physical orientation (gyroscope, accelerometer, and compass) and audio.
In order to turn the first into tracking the phone has to either do the inertial tracking itself (possible but computationally expensive and it diverges pretty quickly unless there have been some major improvements).
The second requires some luck involving an identifiable sound marker some where in the recording then the processing power to cross reference everything.
