ASUS is a very tweak-friendly company: it provides the software tools you need to mess around with the clock speed, etc., without having to go into the BIOS and reboot. The software application is called "AiBooster" which may or may not be installed, depending on whether you installed the various toys on the ASUS CD that came with your motherboard.
My motherboard came with AiBooster, ASUSUpdate (BIOS update utility) and PCProbeII, which lets one monitor various fan speeds and temperatures.
If you're bored with a reliable computer, AiBooster lets you mess with clock speed, CPU core voltage and other things.
In general, ACPI is the Advanced Configuration and Power Interface, an open standard to allow software to monitor and manage various power settings on a computer.