This slide shows that with no BEMF compensation, the voltage in the coil would be quickly close to zero, and hence no current would go through the coil and therefore no torque. The motor would stall. Thanks to the BEMF compensation, the current is stable and the torque remains constant. The BEMF value is linear vs. the speed, the L6470 increases the applied voltage linearly vs. the speed in order to come up with a constant current.