Как я уже сказал: Я непосвящён в чёрную

Как я уже сказал:
Я непосвящён в чёрную магию.
поиском находится Soft Power conditions

Soft Power conditions
1. Your computer motherboard and power supply should meet the ATX specification.
2. The maxium current of the +5VSB(standby) should not be less than 800mA, +5VSB pin is a greatest feature in ATX power supply. the power will supply standby current to some controls circuit of the keyboard, mouse, boot, network, Modem, etc. the details may be decided by your computer supportion. for instance, PowerOn by keyboard may need 10mA current, but WakeUp by Lan may require several hundred mA current, or a larger value.
Now days, the output current of many ATX power supply +5VSB has reached 2500mA, you can find the specification from the power supply brand. the current has been sufficient use for most soft power commonly.
3. Your motherboard chipset support boot except power switch means that it supports the soft boot. This often depends on motherboard chipset and BIOS version.
What are the ways of soft power
1. Power On by Keyboard
You should first refer to your motherboard manual to see if need to set some jumpers on your motherboard, but also may not need, determined by the board type.

Set BIOS to enable Power On By Keyboard, for many BIOS version, this setting may be definition as any key to boot, you can also configure it to a hot key(some keys combination) , fininally, save these settings to CMOS.
Then turn off the computer, be careful not to cut off the power supply completely, you can reboot your computer by any keys or hot keys you have set in the BIOS.
2. Power On by Mouse
Your mouse can be configured as a soft power for many BIOS, here the mouse is the boot entry. Different BIOS may be different in the setting names., access BIOS and check for the details, if you find some settings called Power...By Mouse, this should be the right position. enble it and save the settings, then you can start your computer by double-clicking the mouse.
Note:
There may be some support problems for some type of motherboard on the USB mouse, if so, try a mouse with PS/2 port.
3. Power On by Password Combination
Some BIOS also support booting password combinations, such as a part of Gigabyte motherboard, you can set the MultiKey PowerOn By Keyboard achieve their goals, save the BIOS settings after setup is complete, then you can boot your computer by enter a proper password via the keyboard.
Of course, if you forget the password, you can also start button on the chassis, or the CMOS discharged back to the initial state.
4. Timer Boot
Some motherboards also support the timer boot function, we can set the timer datetime in the BIOS, for example, you can set 8:00am every day, the your computer can boot itself automatically in 8:00am everyday.
5. Wake Up by Modem
Wake Up by Remote ways are often used for remote diagnosis, there are two common ways in the remote wake up. Wake up by modem and Wake up by LAN.
For modems, you may face a internal modem or a external modem, if it is a built-in modem, you must attention to the +5VSB power connection, that should be connected to the modem WOM port, then the modem will have a power standby. but you need no setting if you are using an external modem.
When the modem connected, you can enable the BIOS by setting the Modem Ring or PowerOn by Ring setting up remote boot effect.
6. Wake Up by LAN
Wake up by LAN is also preferred, you can wake up a local machine or a remote host by a professional software. To realize this soft power, your motherboard must support WakeUp by LAN. for some dedicated network cards, if the computer has been powered off. the network cards will be closed, too. it can not get any power from the PCI slot. you may need to connect +5VSB jumper to the WOL socket in the card.
But this is not for all, there are also some motherboard integrated card has built-in Wake Up by LAN feature, then no longer need the connection, just enable Wake Up by LAN in the BIOS. then you can use the software for remote wake-up.
Note:
These settings are not completely off the basic +5VSB(standby) power to achieve the soft power, if turn off the power completely, and turn on again , some booting features may be lost, but not at all(determined by your motherboard and BIOS settings or the default settings).

А это значит что 5vsb подаётся на некоторые части схемы где используется 5в - опционально. для включения с клавиатуры, например.
Ну могу предположить что также для увеличение ампер.

Так?