Copied from the CPUIdle website
www.cpuidle.de/works.html
The CpuIdle Approach
Under normal circumstances the CPU isn't always active but spends much time waiting for the keyboard, harddisk or CD-ROM. What would be more logical than to turn off the CPU for that period? That's exactly what the HLT machine instruction (Opcode F4) does. Whenever the CPU encounters a HLT instruction the clock is halted and the CPU enters suspend mode until an interrupt, NMI, or reset happens. With the advent of power saving microprocessors like the Cyrix Cx486S the HLT instruction elicits an additional benefit. When "Suspend on HLT" is enabled in the configuration register the processor not only stops on HLT but also enters the power saving suspend mode.