Hopefully! you have now grasped the timing for the basic double haul, I'm going to make it more effective - don't give up on this if you haven't; it takes only a little practice when suddenly it all clicks. To begin with, the first time you try out the double haul for yourself, you know, live, outdoors with a rod in your hand and not a pencil, practice the timing given in the previous lesson. Many people find the timing given in this second lesson to be slightly more difficult initially, but easy to master once the basic co-ordination has been ingrained.
In this video I am applying the haul as I'm turning the wrist over at the end of the stroke. If you want to break it down further, close inspection of the Sexyloops videos should reveal that I begin hauling in earnest as the rod butt passes the perpendicular.
When teaching this timing I often replace the word "Stop" with the "Down" in the "Down-up". The Stop is a squeeze stop, for many people it is also a little flick of the wrist.
There are two common faults when you first learn to double haul. The first is failure to get your hands back together for the beginning of the forward cast. The second is getting the hands together but with slack line forming between the hauling hand and the butt ring. It is important to think "smooth", slow down and work on making the backcast no more than a upward flick of the tip.
Some instructors I know teach turning the wrist out slightly so that the reel and rings are out to the side, this can speed up learning the double haul since line doesn't grasp the rod during the pause, but instead shoots easier, enabling your hauling hand to drift up.
There have been numerous discussions regarding the exact mechanics of double haul here on Sexyloops. The double haul has two effects: a greater load on the rod and a direct increase in line speed. One of the best tips in double hauling for distance is to haul as fast as you possibly can.
Please give me feedback on these lessons; to my knowledge this has not been attempted before. Check out this page which shows the Double Haul from another angle.