The trick is for what, just testing? No matter what else that wire pair does, it tells the power supply that a motherboard is connected. There is no motherboard connected to the motherboard connector when that wire pair is jumped.
There is no need for a motherboard to be connected. The power supply turns on the when voltage on that pin is low and it turns off when the voltage on that pin is high. A motherboard, switch, or paperclip all do the same thing they connect the power-on pin to ground which pulls the voltage low. When the power supply is on (plugged in and any hardwired power switch is turned on) it monitors that pin and supplies continual power to the 5VSB rail. When it goes low it "turns on" = supplies power to the other rails, and when it goes high it "turns off" = disconnects power to the other rails.