i know the thread is a little old but hey, valuable info is always worth sharing...
xalted covered hotwiring ATX power supplies but if you're anything like me electricity makes you nervous and a little conformation really helps.
so here it is, diagrams, pictures and all
There are 20 pins on a standard ATX connector. You must first locate pin 14. This pin is very easy to spot since it's the only one with a green wire. All you have to do to make the power supply start is to ground this pin. All pins with black wire coming out of them are grounds. I used a twisted paper clip to make contact between the 2 pins (the green one and any of the black ones). Once your pin 14 is grounded, the power supply should turn on whenever it's switch is on. To know if it works listen to the power supply fan, it will spin whenever the power supply is on.
This is the most basic way to use a second power supply, but it's not very practical if you want to mount it inside your case. You will have to stretch your harm to the back of your case whenever you turn on your system. So the next step would be to relocate the power supply switch to the front of the case. For this, you will need a normal ON/OFF switch and some wires. Simply install the switch between your pin 14 (green) and your ground like this:
here's a link to the page i got this link from with some other options for hotwiring an atx power supply, this is the simplest one and imho the best for our application
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