I found the thread viewtopic.php?f=196&t=2389720
in it ozchrisb describes it as
"There's a line that goes from the rear pulley, down to another ring on the rear line attachment, and then onto the rear line bridle. It's adjusted so that when you pull the bar in hard it really pulls the B/C pulley and the trail edge bridles and the kite cambers up a lot. It's used downwind."
Although the description is not that clear, a picture would be great. It sounds like what you are describing joriws. I interpret it as a different way of implementing a WAC line. They are restraining the ZC relationship. Which doesn't make any sense to me. But if it works heh?
I would have thought restraining AB as in a traditional WAC would be more fruitful and even adding C to the mix. A separate line to trim A independent would produce the best positive camber or hollow profile. But it sounds like they are trying for some weird more mechanical flap like camber.
kitexpert the way a positive camber is introduced is by changing the settings on the mixer. This is something any flysurfer rider is familiar with as they have had adjustable mixers for over a decade or more. As to changing in flight the setting this will require additional line/s to the rider.
As to why you would want to change the settings. Different profiles produce different results. The depower, is not changed* by the mixer. This is "hard wired" into the kite with the positioning of the A bridle gallery, and the ballooning of the kite. Having said that it is possible to limit the depower by preventing the other galleries from going slack. Kites that do that have their reasons.
You are identifying the problems with a particular profile. Positive camber are unstable and collapse etc. But the higher the camber the higher the lift and so drag of course, would seem good for down wind no? and it is. A neutral or negative camber will be more stable better upwind. If you could have both, an advantage in something like racing could be had.
You are right A lines are fixed. With an AB WAC line the load is transferred to your hands but through B pulleys at 1/4 force. So the A mains go slack or slackish depending on the sheeting. This is a different effect. A B become fixed and only C and Z will then sheet in. This increases camber and with it lift. To be effective the WAC needs to be short, which limits depower. That and with the increased bar pressure is probably why flysurfer stopped using them. Gunnar should know he was around when it was removed.
If you were to run a line to A mains and adjust them independently. It would have the same effect as adjusting BC. This would also depower/power the kite so you would need the trim capacity to overcome it.