Its Intel's speedstep tech at work. That?s what it does it drops the multi to reduce the idle clock, to save battery life. Then, as soon as work is required it shoots backup to full clock.
I don?t know if C1E is present on those chips, but this works by windows sending halt commands to the processor which basically does the same thing. Reduces power usage.
It?s a good thing if you're using a laptop (obviously). But not really needed on the desktops. So just leave it, you won't see any performance difference.
If you want to see it in action, just run prime or super pi etc. Open up CPU-z and watch the multi and clock when it is under load from either program. Then watch what happens when you stop the program and the system goes back to idle.