It is the place where the arm of the cabra was attached for maintenance operations. It is not clear to me how the water got to the rueda . There was an outlet with a grid, but this canal led towards a small construction without clear function. It may have hold some kind of a valve. I suppose the canal ends in a second room leaning against the harinero, but without direct doorway to the mill.
Some machinery balances above a deep pit.
I couldn't explore more in depth because of the snow. The mill of Pedro Buil is another simple harinero.