Turbo Boost is a feature of Intel iX CPUs that dynamically increases the clock speed in response to load. TurboBoost only activates when the power use, number of cores in use, and core temperature is low enough. It is designed to provide a boost to single threaded applications running on multicore CPUs. If you have a fully multithreaded video render running, Turboboost likely won't be able to activate since both cores will be in use, the temperature will be high, and the power use will be high.
i.e it enables automatically!
- Simon