When the phone is stationary the sum of the acceleration vectors should be +1. When the phone is rotating (assuming the sensor is off-center) the sum of the vectors should be more than 1 and (hopefully) somewhat constant.
If you look at the decay of that curve, I wouldn't be surprised if that shape is distinctive enough to be used to determine whether the phone is rotating or not.
This is the AccelerometerGraph sample app from Apple.