On January, 18th 2000 Intel introduced the 600MHz Mobile Pentium III processor and with it it's new Power Management technology called SpeedStep.
The original SpeedStep technology, featured in the second generation Mobile Pentium III processors from 600 to 1000 MHz, was rather primitive. The processor was capable of running in two modes, the full power mode and the power save mode. As soon as AC power was removed, the processor switched into power save mode and remained there, independent of the systems CPU load. It was, however, possible for the user to switch it back to full power mode manually.
Indroduced with the Mobile Pentium III-M processors, Enhanced Speedstep added the capability to automatically switch between the two frequencies based on current CPU load.