This feature rotates the fan during stand-by (when turned off remotely) at a slow speed.
Generally, most power supplies halt the rotation of the fan during stand-by.
However, power consumption increases during stand-by by USB devices or CD-ROM operations and the temperature around the +5VSB circuit of the power supply increases soon after the whole system is shut down.
As a result, this could shorten the longevity of the +5VSB circuit or, in a serious case, could damage the circuit.
The 5VSB FAN function reduces heat generation of the +5VSB circuit by operating the fan at a low speed during stand-by (when turned off remotely), improving the reliability during stand-by.