This one is nice and simple. You know those combat ratings that Blizz introduced in 2.0.1? Crit rating, hit rating, etc.? What RatingBuster does is translate those into percentages. This is harder than you may think, because the relationship between rating and percentage varies according to your level as well as the type of rating. RatingBuster does the math for you and makes your tooltips look like what you see in the screenshot. Specifically, it inserts the little percentages in parentheses by the spellcrit and spellhit ratings.