I dont know if this is a known trick or not. But, last night I hooked up my first GPU using extender cable and for the life of me couldnt get it to work in any combination of GPU's, slots .. on my motherboard.
Turns out after further research I discovered some boards require the presence sense pins to be properly shorted by whatever device is plugged into it.
So, I discovered a trick using a wire to short the needed pins in the x1 connector on mobo, it overrides the sense function and will run w/e device is plugged into the slot regardless.
Heres the graphic:http://blog.zorinaq.com/images/pcie-short-schematic.png