This switch is mainly used as a "breaker" switch, not to turn things on or off, which interupts a signal when it is down. When up it lets a signal through. So I guess you could hook it up between the power switch and mobo, and the switch has to be up to turn the comp off from the power switch. You don't need to have a 12V+ to use it.
FYI I bought this switch at Autozone for 3$ and it was the carbon fiber style, Don't go and buy it from Xoxide for 9$. You can get it at any auto supply store like Pepboys, Autozone, or even Walmart.
I used this switch on our riding lawn mower. Since our mower shuts off when the blade is on and you switch to reverse or get off the seat. I took the wire from the sensor under the seat and cut it, then connected both ends to the switch. Now the mower will turn off only when the switch is down. Its kind of a "kill" switch.