I'm not sure that "duty cycle" is the right word in this context. The most common way CPU's save power is by reducing clock speed and voltage when they are not under load. (Intel Speedstep, AMD Cool n Quiet). This of course means less current and less heat. Intel's latest CPU's (Nehalem and later) also have a improved capability to cut off current to cores that are not in use, for example when running an single-thread application on a quad-core CPU.