The shaft and impeller rotates clockwise, and the blades bend back from the rotation. Even if you put it in the other way they would flip back as soon as you run it.
Also the impeller is on an offset housing vs shaft position that causes it to be eccentric. This allows the pump to pick up water in a large volume, and due to the eccentricity, compress this large volume to a smaller volume which increases the pressure and away you go.
So when you install it, one side will be compressed significantly and the other barely touches the pump wall. Vaseline and a second set of hands helps imenseley.
