The opening (B) has a base (2) provided with an air duct (4), and a diffusion element (10) provided with a grill for diffusing air flow in a direction parallel to a longitudinal axis (X-X') of the duct. The element passes from a position to another position by rotational movement around the axis with respect to the base. Rotation of the element with respect to the base implies translation of the element with respect to the base along the axis. The element has members i.e. tabs (110), adapted for cooperating with inclined ramps (200) arranged on the base.
