It is the fine adjustment - though it is now no finer than the 'coarse' part, which is a turret cleat in the middle with a split line to each rack.
At the weekend it was a bit tight to hold the tension on by hand, but what I liked was being able to let it spin back a bit as we gybed or tacked, as well as being able to pull some on without moving.
To make it easier to hold we went for 25mm extensions on this boat rather than the 16mm ones we had before, which gives you another 3:1 purchase because the spindle is 8mm across. Also more area to grip with in your hand.
To overcome the other problem we had on Atum (which was the end of the tiller filling up with rope), we have a coarser purchase, a bigger volume for the rope to wind up in, and we've gone for v thin vectran rope which winds on there.
The master plan is to make a spindle like the picture though. This will hold itself every turn but does not need a cleat.
Sailing with the system we have at the moment was so good last weekend I can't wait for another go!