What they propose is to put a small mobile receiver inside your car and use the car’s aerial to then boost the signal outside the car, giving you better signal and coverage. Because your car is made of metal it can disrupt and reduce the signal that your phone is trying to connect to that is usually outside your car (the nearest mobile phone tower). So by putting the signal inside your car your phone does not have to try communicate though metal, therefore giving your a better signal. It will also increase battery life as your phone does not have to work as hard to cover a usually large distance to get a signal, it only has to cover a few feet. And dont worry about getting more electromagnetic radiation because you have a phone tower in your car, it is a very weak signal strength compared to what you would normally have as it only has to give signal to the few feet of the inside of the car and not cover kilometers that the big towers have to.