[MeltMan]

ATX has generally two switches. One in the back on the psu itself, and one on the case in the front. The one on the back is a toggle to actually physically disconnect the power from the outlet to the psu. The other switch is a MOMENTARY pushbutton switch. It basically just shorts the 5vsb with a ground and powers up the psu. But, there is a catch, the motherboard will have to hold the psu in the on state by keeping the 5vsb shorted with ground.

so simply put, its a normally open momentary pushbutton switch.