To extract the frequencies embedded in the FID, the computer will run the Fourier transformation (FT), which is a multiplication of the FID by cosine functions with increasing frequencies, covering all the frequency range of your spectral width. Esvan and Zeinyeh call these cosine functions "Trial Frequencies".
Case 1: Using a trial frequency that matches with a frequency in the FID, such as 1 Hz: the multiplication of both functions will always lead to a positive value since the trial frequency, and the FID will be positive or negative at the same time, as both have the same frequency. In this case, the total area under the multiplication curve will always be positive (Figure 3a). Also, as the trial frequency has an amplitude equal to 1, the intensities of the original FID are preserved (Figure 3b).