Skip this bit if you want and come back when you've a bit more experience

This increases line speed and overall fly casting efficiency by using the power of both the casting and non casting arms. With the increase of speed, comes an increase in distance and you don't have to make as many false casts to get there.

All it takes is a little hand-eye and hand-hand co-ordination and you should be casting like a professional soon
Basically all you do when you're making a cast, whether forward or backward, is to give the fly line a tug.

Lift the line from the ground (aerialise the line) into a back cast. Get the Rod tip up at Two O' Clock behind you. Wait for the line to straighten and prepare for the forward cast. Non casting-hand keeping still and holding onto the line tightly. Cast forward and as you feel the resistance in the rod, give a short sharp downwards pull (about 30cm) on the line with your non-casting hand. This will shoot the fly line out in front of you with more power and distance. As your tug is completed open your line hand slightly, allowing line to be pulled out, and close tightly to prepare for the back cast.

So that was a single haul. Now for the difficult bit. Give the same tug on the back cast. And there you go. Double hauling like a champion. Well actually it takes quite a lot of practice to get the timing right but eventually almost anybody can do it.
If you can't manage, for now stick to single hauling and try it again at a later stage. You do have to be pretty confident in yourself as a fly fisher if you want to attempt the Double Haul.

One thing to watch out for is hauling when it's not needed. Flyfishers tend to rely too much on the haul once mastered, which can affect his presentation. And after all, presentation is important...