Pressing and releasing the power button on x86 based systems initiates a clean system shutdown and turns the system off. This functionality is equivalent to using the init 5 command to shut down a system. On some x86 based systems, the BIOS configuration might prevent the power button from initiating shutdown. To enable use of the power button to perform a clean system shutdown, reconfigure the BIOS.
On certain x86 based systems that were manufactured before 1999 and are running an older release, pressing the power button immediately turns off system power without safely shutting it down. This same behavior occurs when you press the power button on systems running with ACPI support that is disabled through the use of acpi-user-options.
For more information about acpi-user-options, see the eeprom(1M) man page.