I am quite new Digital Signal Processing .I am trying to implement anti-cogging algortihm my PMSM control algortihm. I follow this documentation

I collected velocity data according to the angle. And I translated velocity datas to frequency domain with FFT. But last step , Acceleration Based Waveform Analysis , It calculated derivative of FFT outputs with respect to time. Outputs are frequency domain how could I calculate derivative of FFT outputs with respect to time , and why does it do this calculation?