From: http://www.overclock.net/a/gpu-and-cpu-power-connections

Let’s look at the pinout and discuss adaptors: The presents of a ground connection at sense A and B are 
used so that the graphics card can detect what types of connector(s) are attached; therefore how much total 
power is available. You can plug the six-pin connector into the eight-pin socket using an offset arrangement,
but I would not do this. Depending on the design of the graphics card, it could either shut down or operate 
in a reduced functionality mode.