The certified flight instructor (CFI) submitted a written statement. He was preparing the commercial pilot-undergoing-instruction (PUI) for her CFI certificate. The purpose of the flight was to work on commercial-level takeoff and landing requirements. After departing, they entered the airport traffic pattern. While on the downwind leg of the traffic pattern, they completed the landing checklist but delayed the landing gear extension because they intended on conducting a 180-degree power off approach. On final approach as they were preparing to extend the landing gear, the air traffic control tower controller told them to conduct a go-around because the landing gear was not extended. They extended the landing gear and the CFI verified three green lights. He then requested permission to land and the tower controller cleared the airplane to land. As the airplane approached the runway, a high sink rate had developed. The CFI told the PUI to add power while simultaneously starting to move the throttle control forward. The CFI indicated that his hand slipped past the PUI's hand, and he reached again to push the throttle forward when the airplane touched down hard on the runway. They aborted the landing and after conducting a low approach to verify the position of the landing gear, they landed uneventfully. A post accident examination revealed rib damage to the left wing. The CFI reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.