A question for riot about Flashing during cast times

What Determines how an ability respond due to a flash during a cast time? Some Abilities like Q Don't allow for flashes during the cast time Some abilities such as Qs allow for flashing but the skillshots originate from the original location, not from where they flashed to Some, such as Q, will originate from the newly-flashed to location. What determines whether flashing will work during a cast time/what effect it will have?