It's well-known that for powering up a standard PC power supply, its PS_ON# line must be momentarily pulled low (i.e. connected to GND).
But how does the motherboard actually turn it off?
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The motherboard doesn't turn the power supply on and off. It controls it between a low power standby, where only the 5v_STANDBY line supplies power, and full output.
What controls these two states is the (perhaps mis-named) PS_ON# line. Logic on the motherboard, powered by the 5v_STANDBY line, pulls PS_ON# low permanently to enable the power supply main output, and releases it high to disable the main output.
There are two converters inside an ATX power supply:
The latter one is entirely independent from the main converter and is always on. This output supplies some of the logic blocks, BIOS and some USB ports. Besides, some motherboards have a feature for powering up from mouse, LAN or keyboard. +5V_SB is needed for this as well.
The main converter can be activated by pulling low the input called PS_ON (green wire) or can be deactivated by either pulling that input high or leaving open. That is what actually the motherboard does. This task is shouldered by a logic block and this block is supplied from +5V_SB.
So, the ATX power supply is never turned off. Only the main converter is disabled by the motherboard.
After reviewing a few (of dozens different PSU) i've here in the bin i see most had a bridge (gray/purple cables) from the SG6105D to an optocoupler through a 100R which enables ground pin from 431A through the optocopuler's led. Unshorting the PW_ON ground connection at the optocupler's leg would cut the signal feedback-loop driving the MOSFETs, halving the large transformer's high current pulsed signal instantly to zero.