This is the device that the rudder servo is connected to. The rudder servo connects to the gyro and the gyro plugs into the rudder channel of the receiver. It is used to sense sudden movements of the tail and it commands the rudder servo to compensate for it.
They say using Gyro (or Dbox) in car, gives Drift control, so drifting is mainly control by the gyro, the driver just points the car. Try to search on youtube, there are some vids about it.