DFT crop the spectrum due to pad 0

In the opencv_tutorials,the example 2.8 Discrete Fourier Transform we pad the image with 0,when we finish DFT we need to throw away the newly introduced values,but i can not understand this statement "magI = magI(Rect(0, 0, magI.cols & -2, magI.rows & -2));" can someone explain this to me,Thanks a lot！