The flight instructor was providing initial tailwheel training to another pilot (who also held a flight instructor certificate). After practicing maneuvers in the local area, they practiced three uneventful landings. During the next landing, as the airplane was slowing down, it started to veer to the left. The flight instructor directed the pilot to apply right rudder, but she overcorrected. The flight instructor then called for left rudder application, but the pilot again overcorrected, so the flight instructor again called for right rudder. The pilot did not respond and the instructor repeated his instructions, with no response received. He then called for a transfer of control and applied right rudder, but described feeling “heavy resistance.” Despite pushing on the pedal, the airplane continued to the left, departed the runway surface and impacted trees. The airplane’s wings and fuselage were substantially damaged during the accident. The flight instructor reported that there were no preimpact mechanical malfunctions or failures of the airplane that would have precluded normal operation.