Before you ever press the power button, certain "always-on" voltages are live on the motherboard as soon as the power supply is plugged in and switched on.
As soon as the PSU is plugged in, it sends 5V standby voltage to the Super I/O (SIO) chip and the Southbridge/PCH Reset Signal (RSMRST): The SIO sends a Resume Reset desktop motherboard power sequence pdf
The desktop motherboard power sequence is a strictly timed, logical chain of events controlled by the Super I/O chip (SIO) and the Embedded Controller (EC) or Platform Controller Hub (PCH). For a computer to turn on, every step in this sequence must occur in order. If one step fails, the sequence halts, resulting in a "no power" or "no post" condition. Before you ever press the power button, certain
If your board won't turn on, technicians typically check these points in order: Is the SIO getting standby power? If one step fails, the sequence halts, resulting
: The SIO chip pulls the PSON# line low, telling the PSU to turn on all main voltage rails (+12V, +5V, +3.3V).