Dual line kites change all that. If you pull the right line the right half of the sail is pulled towards you and the kite turns to the right, or clockwise, and if you pull the left line the kite goes left, or anticlockwise. Very simple and intuitive, but in fact the aerodynamic principles involved are less simple and have been the subject of lengthy discussions on rec.kites.
It is also possible to control the speed of the kite, by pulling both lines together - the harder you pull, the faster it goes, and if you allow the lines to go slack by throwing your hands forward, the kite stalls.
There are three widely available types of dual line kite: the Peter Powell diamond stunter, the delta sports kite, and the dual line parafoil.
Diamond stunters can give a lot of fun and are easy to fly, though one-man launching is only possible by setting the kite on a ramp (easily made on a sandy beach) or a special launching stand, so that a sharp tug on the lines pulls it face-up into the wind. Long tails of polythene tubing are usual but these are for show rather than stability, tracing out turns and loops in the sky.
For a real head-turning spectacle, stack as many identical diamond stunters together as you can afford, attaching the second and each subsequent kite to its predecessor by three lengths of line, each half as long again as the height of the kite, connecting the two kites together by their bridle points.
Several strands of development have already emerged, with their attendant jargon. In particular:
Plans are available for a family of delta sports kites, the "Raaseri", which includes an ultra-light, a medium, and a heavy. Here is the family portrait
Many tricks have been developed with the delta sports kite, and whereas they are easier on a trick kite, an experienced flyer can perform most of them on any good delta. Some involve "groundwork"; in inexperienced hands, most landings are unplanned but experts can land on one or both wing tips in various ways and take off again. Others are built on the "axel": the kite is stalled by throwing both hands forward, and then is made to rotate through 360 degrees, still stalled and floating gently, with a yank on one line. Normally, pulling the right line causes the kite to turn clockwise; in an axel, a yank on the right sends it anticlockwise. A video clip is online, showing Dodd Gross demonstrating the axel.
You often see beginners with delta sports kites who clearly don't know the easy way to launch them. If you are on your own, secure both handles with a stake in the ground. (I always make sure the right-hand handle is on top so I know which is which when I'm ready to launch.) Then, set the kite on its wingtips, downwind from the handles, and with the lines holding it in a position leaning back but not falling over. Return to the handles and remove them from the stake, taking care not to let the kite fall over. When you're ready, a firm pull on both lines will launch it straight up in the air.
Flying such a kite is quite a different experience from a delta, some people finding it very relaxing. Arm movements are greater than with a delta, and the thrill comes from the pull it exerts, as well as the shear speed. The downside is that one-man launching is harder than with a delta, especially on long grass, but the Flexifoil comes with an instruction sheet which explains how it can be done quite easily with a couple of sticks to form a launching ramp. Simply drive them into the ground at an angle facing you and separated by a distance somewhat less than the width of the kite. As you pull the lines, the kite is lifted off the ground, and as soon as the wind catches the underside it should take off.
One thing you must never forget: unlike a delta sports kite, in preparing for a launch it's vital that you only stake down one of the handles, otherwise there is a real danger that it will self-launch and create a hazard.
A major use for dual (or quad) line parafoils is for traction in the new sports of kite-buggying, kite-sailing, kite-surfing and kite-skiing. The Peter Lynn Peel is much favoured in these circles. Stacked parafoils, or single kites of large size can develop tremendous pull - the use of 300lb breaking strain line is quite usual.