Plan view.
It is a 4-bar linkage consisting of two cranks (blue bar, yellow disk) and a connecting rod (green sail). Blue bar rotates on the eccentric of a pink fixed shaft.
Such arrangement makes the green sail present its edge in returning toward the wind, but present its face to the action of the wind, the direction of which is supposed to be as indicated by red arrow.
Output motion (clockwise rotation) is taken from the yellow disk.