A crew was rigging down the flow-line that extended from the bell nipple to the shaker. Two pad eyes providing for a balanced hoist of the flow-line were available to hook up to, but a chain was attached to only one pad eye, resulting in an unbalanced load. The Dresser® sleeve was knocked loose and the flow-line pivoted from the unbalanced hookup. The unsupported end of the flow-line swung down, striking a floorman standing on a lower beam, causing him to fall two meters (6 feet). The fall resulted in three cracked ribs.