According to the Flight Instructor, "the flight was concluding a training flight. The final exercise was a demonstration of an autorotation to a powered recovery. The entry and glide were normal. During the flare the throttle was opened to reinstate the engine power. At this point I was aware that I did not have sufficient power to bring the aircraft to a hover. I leveled the aircraft and prepared for a touch down autorotation. Contact with the ground was level but with some forward movement. The grass runway was soft due to recent rainfall and as a result the front of the skids dug into the runway bringing the aircraft to a sudden stop. The helicopter tipped forward with some force resulting in the mast tipping forward and the blade flexing down. As the blade rotated to the rear of the aircraft it came into contact with the tail boom and severed the tail rotor drive shaft. The aircraft did remain upright and level and I was able to perform a normal shut down. We then exited the aircraft to inspect the damage." The Flight Instructor further stated that "I should have opened the throttle prior to the [flare] in order to give the engine more time to recover before pulling pitch. I was simply late bringing the power back in." The Flight Instructor reported no engine or mechanical malfunction or failures.