Welcome to Tom's Hardware Forums,
When the power is turned on at the back of the PSU, even when the computer is 'off', the PSU provides 5v of standby power to the motherboard. When you press the power button on the case panel, the motherboard uses that 5v power to wake itself to a powered on state and then begins to draw more power from the PSU. Although I would classify them as 'interactive', I guess you could say the motherboard initiates the 'power on' state.
Hope it helps
but how does motherboard gets to know that power is switched on???