Originally Posted by assaulter_99
Well in my experience, it should work and stay on as long as you've shorted the green and black wires. Chances are that the psu you stated was faulty indeed. Faulty psu's I've encountered usually just power up then die in a fraction of a second. (assuming its from a cold start) But regarding your question, the trick to power it on/off without the mobo works. (judging by all that I've used till now)
This is what I would do, and it's is supposed to work on every modern PSU AFAIK (maybe not the specialty ones though like picoPSUs)
even some manufacturers supply little jumper wires for standard PSUs so you can test the hardware in the case with power without possibly damaging the system if it doesn't work right (ex watercooling systems can be powered this way without running the Motherboard, so if your cooling suddenly failed you won't damage whatever it was meant to cool).
however, it probably does need a load and I'm sure it wouldn't like floating without one if it has problems