Basicly throttling kicks in at a set temp. When the CPU reaches this temp it starts to duty cycle. It's not technicaly chaning the clock speed like speed stepping or cool and quiet would do while the cpu is idle or doesn't need full speed. If you have 33% throttling, it's basicly pausing the CPU every 3rd clock cycle to try and cool it down, it's still running at the same clock speed, but has the same effect as if it were running at a 33% slower clock speed.
Prior to going to water cooling, my 3.4 was throttling. I was getting poor performance, and benchmarks were showing my scores way lower than they should have been. I ran throttlewatch and sure enough it was throttling. Now that it's got water cooling, it runs just as it should, but the heat some of these prescotts can generate is rediculous.