When you first apply power to the ATX supply it is possible to get a voltage spike form the unit into the load, or the thing you want to power without the batteries. This what is meant by voltage spike. It may overload the device and do some damage. This can also occur when you turn the ATX off.
You say when I turn on the ATX? What if it's already on, then I connect route output through a switch [external to the psu] and turn it on that way? Does that solve the surge problem?
Alternatively, or in conjunction, could I not add an inductor and/or capacitor so smooth it out? I have a few scrounged caps and various transformers from junked tvs and radios. I can identify the caps, but not the coils. Well, I can measure resistance on coils, not inductance. Another post coming soon on that whole situation.
I can only view these psu surges with an oscilloscope, correct? Which isn't coming any time soon.