How the sensor fusion works in this case?

[Holy Hathaway1]

I have one question. Most IMUs now a days have built in sensor fusion using kalman filters. For instance, Mbientlab-MetamotionR IMU. Now i would be getting angular rate from gyroscope. So how the sensor fusion works in this case (built in sensor fusion)? because if i want to get angle of rotation then i integrate the angular rate and get drift in the measurement.