Thats what I found on the net...
The push button sends a 5-volt signal to the power supply to tell it when to turn on. The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it is officially "off", so that the button will work.
So does it mean the pins where the power button is connected, has 5volts across and the power button either shorts it when pressed or its already shorted and the power button opens the ckt when pressed? My understanding may be flawed, I don't know. I tried to measure resistance of the power button(thinking its simple switch), but I got infinite resistance in both states. I'll check the voltage across the switch and resistance tomorrow.
I hope somebody here knows how this stuff works.