You got another motherboard you can just temporarily power with that PSU? You're supposed to have a load on the 5V/12V lines when testing a PSU so you can't just short the PS-ON line with nothing else plugged in.

The motherboard is always supplied with power via the 5V standby connection from the PSU. The hardware that monitors the power switch is powered via that. You push the button and it'll signal a microprocessor on the motherboard that you want to power up. The microprocessor will then handle the PS-ON signal etc...