Kite Control Systems
Modern control bars are much safer than the ones we started with. Every few years there have been advancements in the safety and refinement of control bars. Many older ones are obsolete, as they will not properly depower a modern kiteboarding kite. But some bars that are quite old, will depower a modern kite.
A modern kiteboarding kite has four lines. In the earlier days, there was one line directly connected to each corner of the kite. Now there is typically a set of bridle lines on each side of the kite, but for this discussion it is still useful to think of one line going to each corner of the kite--two to the leading edge and two to the trailing edge. So the line connections are: front left, front right, back left, and back right.
At a very basic level of understanding, the kite makes power because it forms a shape that catches some of the wind flowing by it. With lines going to each of the four corners, the kite has a cupped shape that catches air. But if you released 3 of the 4 lines, the kite does not cup the wind anymore and falls out of the sky.
Modern control bars let us quickly and reliably release three of the four lines, leaving us connected to the kite by just one of the front lines. That allows the kite to spill the wind it was catching and fall out of the sky with the least amount of power. On a 4-line kiteboarding kite, if you were only pulling on one of the four lines, and the other three were completely slack, the kite would be 'flagged out'. With tension on only one of the 4 lines, you would be pulling on the kite just by one of its four corners. The kite would be able to stretch out and not catch much air moving past it.
To explain this term we use for fully depowering the kite, imagine a flagpole with a flag flapping in the wind. The flag blows straight back from the flagpole. If the flagpole was low enough that you and I could reach the trailing edge of the flag (the flappy end), we could pull the flag to one side until it was catching wind and not flapping anymore. Now we have a sail and can feel the power of the wind in our hands that are holding the flag. If the wind was strong, we might even be bending the flag pole. If we let go of the flag, it can 'flag out' again.
On a modern control bar, we activate a quick release which leaves us attached to the kite with just one line. But we got to this system gradually. Older control bars only released tension on the front lines. With all the tension on the back lines, kites still came out of the sky, but they sometimes still pulled hard as they were crashing out. In 2014, Cabrinha and Airush adopted the single-line system. In 2015, Core and North did also. By 2015, most brands had implemented the modern 1-line safety system.
Control bars that do not use the single-line safety system are obsolete. They provided some level of safety with the (also obsolete) kites they were designed to work with, but they don't give anywhere near the safety of a single-line flagging bar on a modern kite.
You might get away with using one on a trainer kite, but that's the only good use for an old bar other than using the spare parts to make a light-weight foiling bar.
So now you understand that a modern kite control safety system uses a single-line flag-out. But here's a critical detail--the bar needs to flag out enough! The bar needs to slack three of the four lines enough that the kite can flag out all the way. For a big 17 square meter kite, that slack needs to be 10 meters (33 feet). Some bars such as an old RRD only slacked the three lines about 12 feet.
5-line bars: Some kites had a fifth line safety system. The safety depower worked well, and the 5th line helped with relaunching. But the fifth line added more cost, drag, a bit of weight, and a lot more propensity to tangle. 5th lines went away as four-line kites did just as well without the extra complication. Consider 5-line obsolete but not dangerously so. The last 5-line only kites were 2017. In 2018 the few 5-line kites adapted to also be flown on 4 lines.
Still OK because single-line depower:
Slingshot bars going back a few generations: 2010 Compstick with Guardian safety release system.
Slingshot Sentinel 2017
Obsolete because 2-line depower:
Cabrinha 2013 and older. Names: IDS, IDS Quicklink
Single-Line flag out but does not flag out enough:
2018 RRD Global Bar v8: only flags out about 11 feet, leaving kite to death spiral.