The flight instructor stated that he was providing instruction to a private pilot for a tail wheel endorsement. The pilot receiving instruction had previously made three touch and go landings without a problem. During the fourth landing, as the airplane touched down, the pilot receiving instruction began to add right rudder. The flight instructor "immediately" applied opposite rudder and brake to maintain control, and shouted “my plane, my plane." The pilot under instruction was "fighting" him on the rudder pedals, and the flight instructor stated, "let it go!" The flight instructor was maintaining the airplane on the runway with the use of the brake pedal, as evidenced by skid marks, until his seat failed and he slid aft. The pilot under instruction continued to apply right rudder, and the airplane ground looped to the right, impacting the ground, and buckling the wings and fuselage. No preexisting anomalies were noted with the airframe or flight controls.