For double hauling the rod bends and recovers quickly without wobbling and throwing shock waves into the line. I caution intermediate casters from using the double haul because most do not track their rods in a straight line while casting or have other significant flycasting problems. In that case, the double haul becomes a crutch that is employed to cover other deficiencies from poor casting. The double haul is seen by the flyfishing world as the pinnacle of achievement in casting skill. As people try to cross this casting hurdle, they find that coordination, timing and hard work are prerequisites.
Mistakes come from making too long of a pull, really ripping at it.
Your pulls should be in synch with your casting stroke and be of the same approximate length. Short strokes mean short pulls. Long strokes mean long pulls. Try this in practice. A second mistake is to make your downward pull without returning your line hand to a position near your rod hand.