I am trying to repair a plissee blind, that does only fold up until half, then stops, no matter how high you push it. It has a metal-thing holding three metal spirals that can be connected to the plastic drum in the middle. The middle plastic drum has two square-shaped holes where the metal spirals can be connected (see pictures, it's hard to describe). It also has a kind of shuttle that is connected to the two strings pulling the blind. The shuttle is connected to the metal-thing via another string. The metal thing should strongly pull the shuttle to the right, so the blind will be kept up (this is the problem). If you pull the blind down, the shuttle should move left and should be kept left by the weight of the blind. So far I understand it. But now I have no clue how I can make the metal-thing have that strong pull to the right. I can pretty much guess how the left and the right metal spiral should be connected to the middle plastic drum, because I could see some dirty marks on the metal thing, caused by the metal spirals moving around. But I have no clue what to do with the middle metal spiral. Even theoretically, I have no clue about its function, because however I put it, it seems to "do nothing".
Is there anybody out there with a better technical understanding to imagine that spiral spring in practise? I am thankful for every hint or resource, as I could find absolutely nothing online (at least in German; I am missing the right research words in English, as these seem to be kind of special terms).