I too went suction mainly as Jambutty said, the initial action that caused the movement of air is the piston going down. hence as it instigated the whole proceeding I reckon suction is right. Atmospheric pressure was only equalling the pressure so it wasn't pushed in as all,it was dragged across. To me pushed infers that 'positive pressure' is in action.
Well Done Jambutty though!
The ones that made me think were the multiple pully ones.