This is a great forum with great people, thank you all for your advices!
I think that the tips from Dyylan are very helpful and I will try all this. Light wind for me with that kite is about 8 to 12 knots, I'm 69 kg.
I just never had the problem with my old JN kite that the kite drops while flying fast sinus curves. Always thought that when flying sinus curves that I had to use the maximum way from 12 o clock to close to the water and to pull the bar at maximum for faster steering to get the optimal power in light wind conditions. But what Dyylan says makes sense.
The problem is astonishing to me, because the Bolt is very stable when riding waves. It's even possible to surf a wave fast (long button turn) and all the lines have no tension for a moment, but the kite stays at the sky stable like a wing until the next cutback.
Today, the wind was much stronger, gusty and at the limit for the kite size. In this conditions the kite never stalls, but the depower line on the universal bar is very impractical: bad position very near the chicken loop and the safety line. Had sometimes to "ly down" in the water with the kite at 12 o clock for adjusting the power. And the depower is limited due a fixed "stopper ball". Other kiters suggested to open the screws of the stopper ball to make it gliding.
Has anyone tried that already?
In my opinion, Naish has still a lot of improvement possibilities.
The square thingy you can easily take off the line. It is there, I think, for riders with shorter arms. You will get the technique depowering while riding..when you move the kite up, pull the depower down. It should work fine. They changed the orientation of the teeth quite a bit and it made it a lot easier.
A lot of kites needs to be able to breath a little in lighter winds. It is a bit counter intuitive to sheet out in light winds, but it is normal. The alternative would be to let that sweetspot be and have it adjusted so that the sweetspot in lightwinds always becomes the sweetspot. You can easily do that by moving the steering line out one knot under the floaters....but then you will lose a bit on the wave riding capabilities IMO.
This is not really recommended but you can take off the safety line and just use the depower line as your leash connection. If you do that, just know that if your depower lines breaks your whole rig goes.