Nobody really knows how it works. The leading guess is that it adjusts running parameter tolerances. Yes, it's generally helpful in overclocking so long as it is properly implemented by a motherboard and its BIOS.

How to use it? You get a range of positive and negative values. You tweak the values for each core until you find the sweet spot that allows the greatest stable overclock--too much and too little are equally bad. Positive values help OC.