Anything in the BIOS labeled as C1E, C2/C2E, C4/C4E or EIST needs to be disabled. But if the downclocking isn't causing a problem there's no reason to disable it. Your CPU will idle cooler and use less energy.
It clocks up instantly... even on the smallest of loads... you open a program and you'll see it jump back up to normal speed, then once you're idle it'll drop back down...
It's a nice feature if your overclock is stable...