What happen's if you connect the motherboard, then plug in the PSU ? Does it still start even without pressing the power button on the computer case ?
They way I've seen most these turn is
)There's a mini SMPS, that creates +12-17V Aux. power on th primary side, and powers the isolated 5V standby power and the chip on the output side.
)that 5V goes into an opto-coupler circuit, and the computer power switch either connects the 5V at the top, or hooks the botom to GND, to complete the circuit
)On the primary side, the other side of the opto-coupler, is controlling a transistor(s), that turn on or off, the highpower SMPS chip, on the hotside..............the Vcc pin of that chip is either pulled low, for off, or high, so it starts running.