12v and 5v are from PSU from computer.
9v would be back up battery.
The NC switch would represent a power cut
When power is cut from from PSU the relay will flip over to the battery backup. And when power is restored the relay would automatically flip to the PSU. This is not efficient for something that isn't plugged in to mains, for the original source. A resistor might be needed to stop the relay from being cooked. Press the switch to stop power that would come from the PSU's 5v line. You would see it relay switches and the LED would continue to stay on.
