A flight instructor with four total hours as a CFI was training a student pilot. The student made three touch and go landings, during which the CFI later stated the student did not maintain airspeed, resulting in a high sink rate. He instructed the student to conduct a short-field approach with an obstacle. The airplane descended to 25 feet over the runway when its airspeed decayed and it descended sharply. The student pilot did not apply power. The CFI reached over to add power but missed the throttle. He flared the airplane as it hit the runway hard, resulting in structural damage to the airframe.
