@lamp @Zergling_man If a computer is plugged in, it is not off - something electronic needs to be powered up for a non-on/off pushbutton to turn it on after all.

Computers with an (operating) IME or PSP are always executing proprietary software even if the "off" state.

"Wake On LAN" is a feature of some NIC's, where the NIC goes and holds open a UDP connection to someone else's computer and waits to receive back a "power on" packet and boots the computer on receiving one (I reckon it just needs a bit of extra wiring to briefly short the power-on pins, just like what pressing the power button does).

In ACPI S3 suspend, the computer is shut down, except the memory controller is still active and maintaining contents of the RAM.