According to the pilot-in-command (PIC), after touchdown a gust of wind pushed the tailwheel-equipped airplane toward the left side of the runway. In an attempt to correct the drift, he applied right rudder and right brake. The check pilot, seated in the rear, added engine power, which resulted in a ground loop. The airplane continued its turn, about 150 feet off of the runway surface, until the left main landing gear dug into the ground and sheared off. According to the check pilot, as the airplane crossed the end of the runway, about 5 knots above stall speed, the wind shifted, with gusts up to 30 knots. The airplane stalled and descended to the runway with a rapid sink rate. The check pilot advised the PIC to go around and instructed him to add engine power. The check pilot further stated that the PIC thought that he wanted the check pilot to take control of the airplane and raised his hands off the controls and said, "You got it!." By that time, the airplane was off the right side of the runway in the sand with neither pilot at the controls.