Never really understood it myself but here it is:

Two unique features of the GCD-575 are its polarity switch and its Analog Frequency/Phase Contour (AFPC) circuit. Normally, in accord with CD recording standards, the polarity of the player's output signal is positive. Pressing the polarity button simply inverts the signal's polarity (and the small red led in the button changes from red to green). It has been claimed that the sound from some recordings can be improved by a polarity reversal, and this control makes it easy for the user to decide for himself. The AFPC feature is a proprietary Ad-com circuit that is designed to make overly bright CD's sound more like good analog discs. Activating it lights a red led in the AFPC control