Unconventional voltage regulation scheme in the secondary of an ATX power supply, how does it work?
L5 and L6 are partially saturated in normal operation, by the forward DC current that passes through them via both legs of D23.

Sending current through them in the other direction via D30 and D31 reduces this net DC component through both coils, which increases their inductance and therefore their impedance, reducing the output voltage.

It's really a kind of magnetic amplifier.

G36 found this paper that explains the application in detail: "Magnetic Amplifier Control for Simple, Low-Cost Secondary Regulation"