Many fly fishermen consider the ability to be able to double haul as the key to catching fish beyond the reach of mere mortal anglers. To them double hauling is all about achieving distance. But this is an over-simplified view of an important casting technique. The essence and purpose of the double haul is to increase line speed. Increased line speed enables you to cast tighter loops, and tight loops and high line speed give you long casts.
The basic principle of the double haul is very simple: but achieving it successfully is, initially, quite a different matter. To make a haul, simply pull down on the line with your line hand as your casting hand speeds-up and stops. If you do this at the end of the back cast only, it is a single haul. Pull down on the line at the end of the back cast and forward cast and that is the classic double haul. The line hand's actions should mirror those of the rod hand. The length of the haul should be short and sharp, like the short sharp acceleration and stop at both ends of the casting stroke.
To make a successful haul on the back cast, keep your hands relatively close together as you lift the line off the water to make a back cast. As you near the end of the cast, your rod hand speeds up and stops and at the same time, your line hand pulls down and then immediately goes back up to your rod hand as you start the forward cast. As you make a short speed-up and stop with your rod hand, pull with your line hand for a similar distance and then release the line exactly when your rod hand stops. A short, sharp haul - 6 to 12 inches (150mm to 300mm) - will be much more effective than a long, continuous haul matching the length and duration of either the forward or back cast. Long hauls don't necessarily help you cast any further than will a well-executed short haul. Keeping your hands close together really does work: it eliminates slack as well as giving more time and distance between the two hauls.
Start with 30 feet (9.14m) of line outside the tip ring - less if you cannot aerialise the full 30 foot head of the line. Start off by trying to make a perfect but very short haul of 2 to 3 inches (50 to 80mm). When it is done well, even a 2 inch haul will give you a significant increase in line speed. When you are happy with a very short haul, you can start working your way up to 6 to 12 inches, as above.
Any cast can be improved and made easier with a double haul. Even for short or medium casts, a double haul gives you that little bit of extra line speed which means that fewer false casts are needed to extend line. If you are fishing amongst trees, for example, where there is very limited room and a back cast can only be made with a short rod movement and a very short length of line, a double haul can help you get that bit of extra line speed and distance that you need to cast to a fish in the middle of the river or lake. Extra line speed and tight loops will help you cast into a wind. A double haul improves line control and reduces the need for excessive false casting too.
Learning to double haul seems almost impossible at first. It's a technique that, after plenty of practice, you suddenly realise that yes, you have just completed a successful double haul. Once learnt, the technique is never forgotten. But the double haul should only be used when you have mastered the basic casting stroke.
Practice the cast away from the water and the distractions of rising fish. The key to double hauling is rhythm. It is also the hardest part of the cast to learn. It can be helpful to stand more sideways so that you can see what is happening to your backcast as this can help you improve your timing.
Once you have had that "Eureka! moment" and started to master the technique, now is the time to try the double haul for real, on the water.