On the left, the static bridle (shown here as a front-on view of the right wing of a kite) has 3 legs which hold the tow point (green flash) in a static (i.e. not moving) position. These legs are commonly called the inhaul (the line going from the T-Piece to the tow point), the upper outhaul (going from the upper leading edge connector to the tow point) and the lower outhaul (going from the lower leading edge connector to the tow point). On most kites, the two outhauls, or one of the outhauls and the inhaul, are actually a single piece of line to which the third leg is attached.
The dynamic bridle, shown in the middle picture has an extra section, the yoke that connects the tow point to the outhauls. The lengths of the inhaul and outhauls may not necessarily be the same as on the static bridle, but it is possible to construct a dynamic bridle that has the tow-point in the same relative position as the static bridle equivalent.
The key feature of the dynamic bridle is shown in the illustration on the right. Depending on the pressure applied to one or both flying lines, the tow point automatically shifts up or down whilst still maintaining tension on all lines in the bridle. On an equivalent static bridle, one leg or more will go slack if the tow point moves from its "home" position.
This has two effects on the way the kite flies:
In most conditions, a higher tow-point (i.e. nearer the nose) will increase the forward speed of a kite and reduce the turning speed. Conversely, a lower tow point will slow down the forward speed but radically increase the turning ability of the kite. Too much of either is a bad thing, introducing sluggishness, unresponsivness or perhaps excessive oversteer. Designing a static bridle is about finding the right balance of these characteristics and making trade-offs between them.
The dynamic bridle can provide the best of both worlds. When flying in a straight line there is equal (or near equal) pressure on both flying lines. The effect on the bridle is that the tow points are pulled in and up, increasing the drive and improviing the tracking. When one line (say the right) is pulled to instigate a turn, the opposite wing (the left) falls back away from the pilot. The dynamic bridle on the tensioned side (right) shifts the tow point down and out towards the tips, producing a tight spin. As soon as the lines are pulled back to equal pressure, the bridle reverts back to its upper position and prevents the oversteer that would normally be found on a low set bridle.
Many cutting edge freestyle tricks rely on getting in and out of positions where the kite is no longer facing the flier. Axels and Flat Spins, for example, have the face of the kite pointing down towards the ground. In these positions, a static bridle would normally have one or more legs loose. When the flier tries to bring the kite out of the move or directly into another, the first few valuable moments when a line is pulled or popped are spent on taking up the slack in the bridle.
A dynamic bridle has a wide range of positions where all the bridle lines are tight. This means that the flier's actions are transfered directly to the frame of the kite without putting all the pressure into a single bridle point (which often causes the kite to fall out of the move in a somewhat ungraceful fashion).
The diagram above shows this in action from a side-on view. The blue lines represent the "normal" bridle position. When the kite is tilted forward, the upper outhaul on the static bridle becomes slack (left) whereas the dynamic yoke automatically shifts to a higher position, keeping all bridle lines tight (right).
The most striking example of this benefit of the dynamic bridle is when trying to do Multiple Axels and Flat Spins. With a static bridle, the timing and force of each consequetive "pop" is critical to get the kite to remain flat but continuing to revolve. A dynamic bridle gives a much larger margin of error, to the extent that it's possible to keep Multiple Axels and Flat Spins going almost continuously, or at least until your flying lines start binding (which happens at about 25 revolutions, I can reliably inform you).