I always saw the thresh as the ceiling the actually parameter of the comp that nothing should pass meaning wherever you set the Thresh it should be above the signal not below it.../QUOTE]
In fact, the threshold of a comp is the opposite of a ceiling. It is the "floor", from which anything above will be compress.
The ceiling is for a limiter. If you put your threshold at 0 db, nothing will go over.
With a comp. using a -10 db threshold and a 4;1 ratio, it just mean that anything going passing the threshold will be compressed in a 4;1 ratio.
Hope it help you understand.