If you're a fire mage and you have the talent Hot Streak, which basically is if you crit twice in a row you're next pyro is free.
well if you use something like scorch, crit twice you get hot streak, then once you have the buff, crit with scorch or fireblast twice and right as you know you'll crit shoot the first pyroblast and you'll get the hot streak buff again right as your crit on the 4th scorch registers allowing you to pyro again a 2nd time.