The field between the poles changes uniformly. With a Linear Hall sensor, angles could be read on say a non-ferrous caliper where one arm has the magnet mounted on it and the other arm holds the sensor that moves along the magnet as the caliper opens and closes.
The voice coil runs current through itself to make a certain strength magnetic field inside of the loops. The magnets will try and center the loops where the magnet field average matches the field in the loops, it pushes strongly -- the drive heads move track to track and settle in a fraction of a ms.