(as the above doesn't provide any means of signalling the "off" - two-channel switch where one channel runs the power so the device is powered up when the switch is pressed, the other pulls up (or down) a pin to signal the button is pressed. On power up, the device turns its "independent supply", i.e. through the transistor, ignoring the button. On power-down, pressing the button doesn't change a thing with voltage but signals request for switching off.