|All PCs, Laptops and Desktop will shut down if the power button is held for four seconds. This is under the control of the BIOS and is not a controlled shut down, it just removes the power to the motherboard in case the of a total sieze up so that Ctrl-ALt-Delete doesnt work, which is also under the control of the BIOS.|
It reverses the process that started the computer running in the first place in that out the power supply into standby mode and there it stay until the power button is pressed again. This is all done under software control.
The "Do Nothing" only refer to the a momentary pressing of the button and if set to shut down, will do a controlled shut down that same as clicking shut down from the start menu.