"Thermal Throttle Mode"

I've an old P6SBA which has a bios setting for "Stop Clock Ratio"
when in "Thermal Throttle Mode" (TTM).

What exactly is this? Does TTM only come on when the CPU overheat
temperature is reached, or is it something related to what CpuIdle does
(ie HLT the processor when it's inactive to save power and therefore
lower it's temperature)?

Any info gratefully received!

Phil.