This is probably such an old system that it has a power button that is not connected to the motherboard but is directly between the mains power and the PSU. In that case releasing the button would mean that the server is just depowered just like you hold the button for a few seconds on a modern system.

This is also why those computers said "It's now safe to disable the power" after shutting down.

Nowadays holding it for 5+ seconds still turns the system off, I wonder if this is a BIOS or a hardware configuration. Probably it's the PSU's logic, to power off if the 2 pins are shorted for more than 5 seconds.

On old AT systems (the ones where Windows 9x would show "It is now safe to turn off your computer"), one could actually press and hold the power button and the system would stay running. And when you're bored you can also quickly move your finger off the button and jab it down again (this would flip the switch back to on), and if you're quick enough, the system would not see that there was a power interruption.

Indeed the old AT power button was a mains (120V) switch, with thick cables going from and to the power supply unit.

>Nowadays holding it for 5+ seconds still turns the system off, I wonder if this is a BIOS or a hardware configuration. Probably it's the PSU's logic, to power off if the 2 pins are shorted for more than 5 seconds.

It is BIOS+Hardware (the PSU is not involved). As a matter of fact to "switch on" a ATX power supply (not connected to a motheboard) you normally use a paperclip (or a short piece of cable) to connect the green with any of the black see:

https://forum.overclock3d.net/showthread.php?t=394

The whole point is that (unless the PSU has a mains switch and it is turned off) an ATX power supply is always partially ON, powering (parts of) the motherboard at all times (this allows for such things as Wake on Lan or switch on via CTRL+F11 or dedicated key on the keyboard).

Without ACPI (which NT4 was) the ATX power button was handled entirely by the BIOS (I think in SMI code). With ACPI the power button (when you don't hold it for 5 seconds of course) was handled by the OS.

This was before the power button function was fulfilled by the OS.

Rather, closing the button circuit (ie, pressing and then releasing) will directly shut off power on the motherboard, the OS cannot prevent this.

ACPI wasn't guaranteed to be supported back then, as it was a newish thing at the time this happened.