"CPU PLL Voltage: Sets the voltage for the internal phase locked loop. The role of the PLL is to ensure that the output clock of the internal processor frequency synthesizers maintains phase coherency with the reference clock signal (supplied from a clock generator located within the PCH).
Unlike Nehalem/Gulftown architectures, Sandybridge can benefit from a small increase to PLL voltage when running higher CPU multiplier ratios (hence CPU core frequency). As always, caution is advised with regards to over-voltage on this rail – personally I don’t use any more than 1.85V and that too when processors are sub-zero cooled. Excessive PLL voltage can degrade or kill a processor quicker than overvoltage on any other voltage rail."