hacking into a computer power supply

Exactly what ps? You may need to have power draw from both 5v and 12v lines. Thus, if you are trying to power a 2.5" drive with it, you would only be using the 5v and would need something to put a load on the 12v.

Short the power on lead, usually the green wire, to ground (any black wire). The supply should turn on. If it's an old supply it may need a load of some sort to run stable.

Yeah, usually the green, but not always. It's not a good idea to rely on the colours alone. Compaqs and Dells especially are notorious for non-standard PSes. It would be safest to try and Google a spec sheet for the PS if possible.

Shorting any other wire other than +5VSB to ground while the power supply is in the off state will have no effect. Shorting +5VSB will cause a brief spark and then shut down the supply. There is no safety issue though as 5V isn't enough to hurt you and 5VSB is current lmited.

Here's the official ATX color coding spec (scroll down):

http://www.pavouk.comp.cz/hw/en_atxps.html

Shorting any other wire other than +5VSB to ground while the power supply is in the off state will have no effect.

Ahhhhhh..... Good point

/slaps forehead