It is a FM Transmitter, It is more a Smart Emergency Battery Cover!
Super thin and light, it is the most thin and light battery cover for iphone 4 in the world at present!!!
1.With Smart monitoring system inside
2.With smart power charging management, monitoring circuit and timing function.
3.With automatically power off function when the mobile is charged enough.
4.With battery capacity indication function in different capacities at 20%, 40% 60% 80% 100%.
5.With 4 FM transmit frequencies: 88.1/88.5/88.9/89.3MHz
6.With FM transmit frequency memory function.
7.With auto power off function in 60 seconds when there is no audio input. 
8.With supporting Iphone 4 data Synchronization function.