Background on LLC:
For those of you who don't really know what LLC is: LLC was a featured added to motherboards several generations ago to combat vdroop. Vdroop is a drop in voltage supplied to the CPU as load increases; basically when you go from idle to load, the voltage would decrease. Given the small voltage tolerance that overclockers are working with (increased voltage is proportional to the CPU frequency/multiplier that an overclock can achieve), a droop in voltage applied to CPU can make a theoretically stable overclock unstable (dropping the voltage below that required to achieve the set frequency). LLC applies additional voltage to the CPU to combat vdroop so that when switching to load, there is sufficient voltage to keep that frequency stable. So LLC is great and you want to turn it on? Yes, but...
For most modern motherboards, there are different levels of LLC that you can set in your bios. At certain levels of LLC (these may be different for each motherboard), the LLC can overcompensate for this vdroop, and actually apply vboost. Vboost is when the voltage actually supplied to the CPU is above the value that you set in your bios. This can be a nice way of ensuring that your overclock will be stable, but you have to be careful, because each CPU has a death voltage (the voltage where, if applied to your CPU, it will likely die). If you are toeing the line near your CPU's death voltage to try to squeeze every last MHz out of your overclock, LLC can bring your actual voltage above this level, which is a great way of killing your CPU (or making it degrade much faster). So although LLC is great for overclockers, it should be used with care, because you may just end up killing your CPU.