When troubleshooting PC builds, it is common to power up the motherboard outside of the computer's case. This enables you to see any diagnostic information given by the board, and makes it easier to add and remove components such as memory and PCI cards. Most motherboards do not have a power switch on the board and instead use the switch installed in the case to turn on the computer. To turn on the motherboard outside of the computer's case, you need to mimic the action of the power switch and connect the relevant header pins together.
Read the manual that came with the motherboard to locate the relevant connector and pins for the board's power switch. Two pins are used for the switch connection and these are normally labeled "POWER SW" or similar.
Connect the two motherboard power switch pins together briefly, using a screwdriver or similar conductive metal tool. This provides the signal to the motherboard to switch on. The pins are all low voltage (five volts or less), so there is no risk of an electric shock.
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Turn off the motherboard by connecting the power switch pins together again and leaving them connected until the motherboard shuts down. This mimics holding the computer's power switch in and signals to the board to turn off.