It dosent overclock, but rather allows for it better. I lets more power into the board, increases stability ect. It allows for better, more stable overclocks and I would recommend having it on performance all the time.
I've set mine to Optimal mode and showed that my CPU (Core i7 4820k 3.7GHz turbo boost up to 3.9GHz) is overclocked to 4.5GHz. I had to set my power options in windows 8 to High Performance mode to always stay on 4.5GHz