Turning on a salvaged computer power supply (Apple 8100 - SMP 220DB 8100 Series)
Ok - I figured it out. This may be too narrow in scope to be of use to anyone but here is what I discovered (based on this page - reproduced below).

I had foolishly thrown away the motherboard cord (hadn't even noticed it) so had no cable color reference. With trial and error using a resistor and some alligator clips, I was able to discover the electronic switch pin, the +5v trickle pin and a ground. Note: This is reference to the pins in the motherboard socket connector (not shown in the photo above) and not to the leads shown in the photo. Those are the power outputs.

I guess a two position toggle switch will work to toggle between the +5v pin and a ground - though I don't know if leaving the switch pin continuously powered while the supply is running is a bad idea.