Citation :CPU VTT is the most important setting for stable overclocks. The CPU VTT Voltage sets the CPU termination voltage, allowing for a 1.55V maximum. Using this voltage in combination with the CPU VCore voltage setting can greatly enhance the system stability while overclocked. The system memory reference voltage is split between settings, the controller reference voltage and one option each for the two memory channels. In all cases, the base reference voltage can be set to a maximum of .03V over half of the set memory voltage. (Taken from HardOCP)
"In summary, the CPU VTT controls the CPU termination voltage. Based on Kirmie's observations, it reduces the number of false electrical signals by providing a form of "ground" to pull out these false currents. ." (Credit to Kirmie for the find) My experience with this board is that the max setting (1.55) provides the best results. I have not had a single crash or temperature variation as a result.