That is Intel's Speedstep. Reduces CPU voltage and clock when CPU usage is low to save power. Gives you only the performance you need as you need it. Doesn't help idle a ton though as Intel already gave the chip the capability to turn sections off, helps more in the intermediate ranges.
(And yes, it is in the BIOS and should be able to be deactivated if you wish (no idea why you would want to unless OCing) under advanced core options (or the like)).