In "Auto", the adapter lets the BIOS read the CPU pins to find what voltage to use for that processor. Alot of the older slot motherboards with old BIOS would not go low enough voltage. The PentuimII ran around 2.1 volts. For the newer chip processors which run around 1.7 volts, the adapter lets you adjust the voltage to the processor lower than the BIOS would. Your BIOS would still read that it is putting out 2.1 volts to the CPU but the slot adapter jumpers would be cutting the voltage down to what ever you set the jumper to so that you would not fry your CPU.
If you have a new BIOS that can detect the processor your running, then I would leave the slot adapter set to "auto" and let the BIOS do the controlling of the voltage.