It's an AMD feature to reduce noise in your system. When the CPU is at idle the motherboard will dynamically lower the processor speed by decreasing the multiplier. When the processor runs slower it will, in theory, run cooler. When the motherboard detects the CPU is cooler it will lower the fan speed and as a result noise from the fan will be reduced. Hence Cool and Quiet.
When a CPU intensive application is fired up, like Doom3 or Half Life 2, the motherboard will raise the multiplier back to it's peak setting (for my 3500+ that's 11) to make the CPU run fast enough to adequately run the game. When you shut down the game, the multiplier gets lowered again to throttle your CPU.
In my experience, the multiplier will change from 11 to 5 at idle which causes the CPU to run at 1GHz instead of 2.2.
Anyone else feel free to correct me if I'm wrong, but that is my understanding.