The pull of the pole is perfectly balanced by the push of the zero, resulting in a gain of 1 (unity) across all frequencies.

[ a = \frac\tan(\pi \cdot fc / fs) - 1\tan(\pi \cdot fc / fs) + 1 ]

If you are looking for a commercial alternative with a dedicated graphical user interface, you can check out the popular Disperser by kiloHearts, which utilizes the exact same phase dispersion principles. AllPassPhase VST - GitHub

While traditional equalizers (EQ) change the loudness of specific frequencies, AllPassPhase tools change their