I've got a Silverstone Decathlon 800 watt that i tried to jumpstart (connecting the green wire with one of the black ones at the 24 pin atx connector) but the PSU acted wierd.
It gave a clicking sound and gave way too low Volts. (measured on a molex connector)

I even tried putting some load on by connecting 2 hhd's and some fans--no love.

Thinking that this psu was broken i returned it.

Now i've got a new one that i tried to jumpstart the same way and it's acting similar.

Completely clueless i decided to connect the psu to an old mobo, connected the power on headers and presto... it worked!

Is this normal for this type PSU? I've never experienced this before.