According to the pilot-in-command (PIC), after touchdown, a gust of wind pushed the tailwheel-equipped airplane towards 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 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, on heading and about 5 knots above a stall, the wind shifted to the airplane's port quarter, with gusts up to 30 knots. The airplane stalled, and "dropped to the runway with a rapid sink rate." The check pilot attempted to advise the PIC to go around "by adding power, but he thought I wanted control of the plane and threw up his hands and said, 'You got it!'" By that time, the airplane was off the right side of the runway, "in the sand and no one in control."