According to the flight instructor, the flight departed for the purpose of providing instruction to the commercial pilot in the right seat, who was working toward his flight instructor certificate. At the destination airport, the flight instructor demonstrated a “power-off 180-degree approach to landing,” then the commercial pilot successfully demonstrated four of the same approaches. On the fifth approach, the commercial pilot “misjudged the landing flare” which resulted in a "firm touchdown." The flight instructor "attempted to pull back on the stabilator control" but was unable to apply enough back pressure to prevent a firm touchdown, due to the force being applied by the commercial pilot. Upon touchdown, the flight instructor executed a go-around maneuver and then flew to the home airport where they landed without incident. Examination of the airplane revealed substantial damage to the outboard and inboard ribs. The flight instructor stated there were no mechanical malfunctions prior to the hard landing.