
I've done some ASIC and digital design, but not any CPU design.
It would seem to me that on a 486, the FPU is not being used, most of
the cache is not being accessed, the off chip buffers/drivers are idle,
the multiplier isn't multiplying, the barrel shifters aren't shifting,
microcode isn't microcoding, etc.  This means transistors aren't
switching which means less power dissipated (in CMOS), which means
less heat.\

From what I understand, the Pentium shuts down those sections of the
CPU which aren't being used in order to cut down on heat/power.

