Yes it is yaw (the rotation around the z-axis). The reason why you can not measure yaw with a accelerometer is because the gravitational force from earth does not change when you rotate the IMU horizontal. In theory you will not measure any difference at all, in pratical however you see the acceleration from the movement. I use the z-value from the accelerometer together with pitch and roll to calculate the force vector (this is theoretical 1, but in practice there is a small difference). I also use the z-value from the accelerometer to know when the IMU is tilted more than 90 degrees to both side, so I can get 360 degree resolution instead of only 180 
- Lauszus