This system is based on ideas and systems that other kiters have kindly shared.
Thanks to them.
Overview of leash
The bungie leash is the standard slingshot leash with one knot in it to keep it tight even with the trim pulled down.
When you spin the bar the leash sometimes wraps itself around the depower a little but as soon as you pull the bar down the shackle spins and lets the leash untwist.
You could probably eliminate this by putting a plastic sheath/pipe around the leash but it isn't a problem for me, the leash is always free I've found.
I used 27m frontlines and 23m backlines because I like long leaders on the backlines but you can also use a line set of the same length, the leaders of the brake lines are more or less the length of the depower + trim + Y so you can use a longer trim and/or Y if you want longer leaders on the backlines, this will also make the leash longer though.
Leave some knots on the backline leaders so you can lengthen the backlines to trim the kite down in strong wind because you can't shorten the front lines anymore except with the trim.
After a rotation you get this before you spin the bar back to untwist the lines.
I haven't got a stopper ball on the leashed line because the ball would be above the twists and might not pull through properly. There are no leaders with knots on the leashed line for the same reason.
Spreader bar and shackles
I found that one line in the wichard (unless it's threaded through the back like the flimsy standard line I ditched immediately) can move and you can find it at the bottom of the trigger where it's very hard to release the shackle. To solve this problem I used one piece of line through the bottom of the wichard, (where I don't want to apply force to trigger it) and put this through a bit of plastic pipe, the leash shackle goes on this, it's tight so the bottom of the trigger is always occupied and you can't try to pull here when you trigger the shackle. Then I threaded 3 pieces of the same line through the shackle in front of the trigger
(above the bottom fixed line the leash is on). There are knots at both ends so you can trigger it whichever way it's facing and puling on a knot means 3 lines are activating the trigger.
I used a small plastic ball on the leash shackle so I could feel the difference between the knots and ball without having to see them. The ball and knots are small so they don't catch in the spreader hook causing accidental releases.
I tested both shackles for release with my own bodyweight, probably not enough, aparently a kite can put as much as 4 times your bodyweight on a shackle. The wichard 2673 releases really smooth when you pull a knot towards you (but not by pulling down, sideways or forwards) with all 3 pink trigger lines pulling on the trigger, releasing every time from the plastic depower. The side release shackle releases from the steel ring although this needs more force when under traction. To release the wichard you pull one of the knots towards you, which seems quite a natural movement, it releases immediately. To release the leash shackle you have to pull down hard.
Y with swivel to let frontlines twist
The leashed line is free to pull through the ring, no knots, no stopper ball.
Be carefull when making the Y, at first I just looped a piece of leader line through the swivel on the trim but then I noticed that the leash might catch in the loop of leader between the swivel and the knot where the top arms of the Y branch out. Watch for this and anywhere else the leash could catch when you release the depower.
I've tried this on land and in the water and it seems to work fine but that doesn't mean it'll work fine for you, feel free to ask anything but please test your own system on land and in light wind (both under traction) before trusting it because different parts act differently.
Please note that I am not doing a shitload of rotations yet and if you only do tricks rotating one way and do lots of them the Y and swivel may not keep up with you and keep the fronts untwisted so watch the Y and if the leash ring and front flying lines start twisting together start doing tricks the other way or spin around in the water to untwist them because the leash might not work like this. A better swivel will probably spin better.
All the best winds