its on a timer but during dive-bomb phase it gets interrupted, so at times you'll see a DS before twisters, and vice versa. its roughly 30-32 seconds apart (I've heard early as 28 seconds as well). use the duty finder timer, or even a macro to remind you. make sure around the 25 second mark, you have 5 stacks of wrath and are ready to IB the DS each time.