You literally turn a corner and go from zero gradient to 10% plus and that holds for the next 4K. This is the steepest climb of the 3 I have done.
The steepness came home to me as I was descending and I got onto that 4K. As you turned each bend I would say to my "s... did I ride up that". It was like a drop, not a descent. No such experience on Tourmalet or Ventoux.
There is simply no rest on this climb apart from about 500 metres about two thirds of the way up.
I was gone at the end of the 4K. It was then a question of counting off the bends and getting to the next one.