The flight instructor reported that he was providing tailwheel instruction for the pilot, who was in the front seat of the tandem-seat airplane and did not possess a tailwheel endorsement. Following multiple landings earlier in the flight, the pilot was practicing a high-speed taxi in tailwind conditions. Upon reaching about 20 mph ground speed, "directional control began to be lost." Subsequently, the flight instructor applied brakes, and the airplane nosed over. The vertical stabilizer and left-wing lift strut sustained substantial damage. The flight instructor reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. An automated weather observation station at the accident airport recorded that, about the time of the accident, the wind was from 170° at 9 knots. The flight instructor reported they were taxiing north on runway 31.