The flight instructor reported that during an instructional flight, while on short final, the headwind dissipated and the airplane started to slow and sink. He instructed the pilot receiving instruction in the front seat to add power, but she did not complete the action immediately. After realizing the action was not complete the flight instructor added full power, but the airplane continued to descend at an accelerated rate and touched down hard upon landing. Subsequently, the main landing gear collapsed and the fuselage was substantially damaged.

The flight instructor did not report any mechanical malfunctions or failures with the airplane that would have precluded normal operation.