I'll try, although a book by someone like Lefty Kreh or other casting guru would explain it better and with pictures. It's just a left hand little tug at the end of each casting stroke that increases the line speed. Most people over do it or mis-time it and don't get any benefit out of it. If you are paying attention to line tension with your left hand already you may be almost there now.
It's like patting your head and rubbing your stomach at the same time at first, then gets to be second nature.
Mel Krieger's video "The Essence of Flycasting" has one of the best sequences on the double haul that I've seen. A tip that really helped me many years ago was a remark from one of Arnold Gingrich's books. He said that the sound of the line going through the guides during a double haul (and cast) was like a wolf whistle. He described the sound as "Wheet-Whee-oo"; the "wheet" being the haul on the back cast and the "whee-oo" being the haul and release on the forward cast. Of course, this doesn't address feeding the line into the back cast (what Krieger calls the "up" part of "downup"), but I had read lots of descriptions of the double haul (this was w-a-a-a-y before the day of videos) and the essential timing had always escaped me until then.
This is something you pretty much have to see. The suggestions about getting a video are right on. the other option would be to get someone to show you. The better flyshops should indulge you on this, provided they have an empty parking lot to use.
The point of the double haul is to increase the line speed during your false casts. It enables you to carry more line in the air, but more importantly, it helps you shoot a lot of line on your final forward cast, making it possible to make a cast much longer than the amount of line than most of us can keep aloft with any technique (using a single handed rod). Here's the basics: As the rod is loading on your backcast (timing is everything and it can't be "taught;" it has to be "learned;"), give a smooth sharp pull with your line hand (one tip on timing: remember that you're not using the pull to create line speed, the rod will do that, the pull merely helps to accelerate the line). Make sure that as the back loop unfolds, you return the line hand back up to the rod, so that you can repeat the process on the forward stroke (you need to do two hauls to make it a double haul). At this point, you can slip some line into the backcast before you start the foreward cast, but that can make the timing even trickier. Repeat on the forward cast (this is when I ususally slip line into the cast.
The easiest mistake to make double hauling is to keep it up one too many strokes. Most guys do a fine job until it's time to let go, then lose all their line speed on the final forward throw, often because they've slipped too much line into the cast, and have gone beyond either their own or the rod's capacity to handle the amount of line in the air. You have to have confidence in your, the rod's, and the double-haul's ability to shoot most of the distance you're looking for. Two or three complete falsecasts is all it should take. A sixty or seventy foot cast shouldn't require more than thirty or so feet of line in the air. A really good double haul will shoot fifty feet of line off the deck of a boat or a stripping basket, and give a satisfying little tug at the reel when everything is all done.
I've left out some of the finer points: you need to open up your stroke a little, especially when you're feeding line or shooting your cast. Like I say, you really need to see it (and not necessarily mine; you don't need my bad habits). And try investing in a Wulf Triangle Taper.
Take a look at www.flyanglersonline.com look for James Castwell, at the bottom of his page is a link to previous articles... under 2nd Qtr. 2001 is an article entitled The Double Haul. Good video sequence of the cast.
Checked a book out of the library about this subject. As it was written it looked like it was kinda hard to understand. After I read it and tried to do it it seemed awkward, so I just forgot about it. Maybe if I saw it being done I would change my mind. Jim
Unless you've got a very good basic casting stroke to begin with, the double haul will not be the panacea for distance most expect. If your best cast is under 60-feet, you'll probably never achieve 80-feet just by employing a double haul. As Ray said, "...The point of the double haul is to increase the line speed..." The result of the increased line speed from the double haul is a tighter loop. In most cases, the double haul in and of itself will only give the average fly caster somewhere around 10 feet additional distance. However, fly casters who have refined (read perfected) their basic casting stroke can easily reach or exceed 100-feet with a standard WF line.
In the words of George V. Roberts, Jr., author of the video fly-casting program Saltwater Fly-Casting: 10 Steps to Distance and Power (http://www.whitemouseflyfishing.com)"...The most distance and power you'll ever add to your cast will come through refining your basic casting stroke: learning to load and unload the rod properly, learning to form tight loops, and lengthening your casting stroke when you're casting distance."
Only when you have perfected your basic casting stroke will the double haul be of any meaningful measurable benefit to you. I'm certain Mark Sedotti, Cathy Beck and even Lefty himself would all agree with that statement.
i was feelin' pretty good about my double haulin' until you said your getting a 50' shoot from 30' of line in the air. Now, when i'm fishin' coho off the beach over 5' waves into an October gale i was,'til now, quite ecstatic with the extra 7' i was getting on my double haul.
I know a stripping basket will let me shoot out alot more line but i've got unreasonable issues with them; and i'm way too disorganized to make tidy loops in my retrieving hand that i know will shoot better; and when i collect the loops in my mouth i usually ... duh .... forget to spit out the line which invariably whips my face when shoooting it.
So i connected four different types of line together; a section of amnesia mono (that really shoots) to a floating section of line which is connected to an intermediate clear sinking section which is then connected to a full sink head. (The intermediate and the full sink head are connected with loops so that one or both can be removed if it's necessary to match specific conditions).
I should have been more careful to keep from implying that I'm shooting any 50' feet of line off the beach. (I did say a "good" double haul, not necessarily "my" double haul, which doesn't always qualify by a longshot.)
However, from the deck of a boat, I can fairly consistently get 80' total distance. I know when I've got 30' of line in the air when I hear the loops connecting my running line to my shooting head go through the guides. Because the shooting head is so much heavier than the level running line, the cast falls apart pretty quickly if I try to keep false casting with more that a few feet of running line out of the rod. So it's trained me to "shoot" most of the distance in my casts. (As the name implies, you might expect a "shootiing head" to shoot better than a conventinal flyline, although people have told me that my forward taper flyline is the smae profile as a shooting head hard-spliced to a running line. I don't know. I cast farther with my head; maybe it's a placebo.)
I was trying to make the point that a lot of guys ruin all that hard work of double-hauling by trying to carry too much line in the air (I do it all the time with a conventional line or my steelhead setup). I'll get great line speed going and these cool bullet loops flying back and forth, and then somehow lose it all on the final stroke, the whole mess piling up twenty feet in front of me, if I'm lucky (if I'm unlucky the whole mess piles up against the back of my head.) It comes from trying to carry more line than I really can.
I'm not a partcular genius when it comes to distance casting, double hauling or otherwise. But I did finaly figure out that if I let go of the damned line on the second or third forward stroke, when I was still generating line speed, all that pulling and pushing and timing (the whole patting your head and rubbing your belly thing) would actually pay off.
So OK: from the beach (or steelheading) I'm usually good for 60'-65'. On a good flat beach (or a steelhead pool with good wading dynamics)with a favorable wind I can manage 70'-75', but generally not for very long. I am still shooting 20'-30' of line though, depending on what kind of line I'm using. On the beach I have to use a stripping basket because I use an intermediate running line. For steelhead I can forgo it because I'm using a conventional floating forward taper flyline looped to various sinking tips. If I'm in a good spot and I'm doing it right, the double haul will pick the floating line off the water. I used to do the loops in the mouth thing too, but I could never quite get it either.