The flight instructor reported that he was providing instruction to a student pilot in the helicopter during a cross-country flight. During the flight, the instructor asked the student to perform a "land as soon as possible" emergency procedure. The student pilot approached the hillside landing site from the south. The wind was out of the west about 8 to 10 knots, which resulted in a left crosswind for the landing. The student pilot reported that, during the approach, the instructor told him he "needed more left pedal." The helicopter descended and decelerated below effective translational lift (ETL). According to the Federal Aviation Administration's (FAA) 8083-21A, the Helicopter Flying Handbook, pg. 2-20, para. 2, ETL occurs between 16 and 24 knots. The student pilot reported that, "We descended below ETL, maybe 10 feet off the ground and still descending. By this point we were what I perceived to be straight, and the instructor took the controls. From what I could tell, he used forward cyclic and left pedal immediately. It was too late." The flight instructor reported that, "As we came closer to our landing spot and began to slow down, I felt that the nose of the helicopter was not pointed far enough into the wind (we were out of trim). I took the flight controls right as we slowed below ETL. The helicopter started to develop a hard right yaw and I immediately gave full forward cyclic." The helicopter developed an uncontrollable rapid right yaw and spun about two revolutions. The helicopter touched down on the skids and rolled onto its left side. The helicopter sustained substantial damage to the tail rotor drive shaft and the main and tail rotor blades. The student pilot and flight instructor reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation. According to the FAA's Helicopter Flying Handbook (FAA-8083-21A), the Helicopter Instructor's Flying Handbook (FAA-8083-4), and Advisory Circular (AC) 90-95, "Unanticipated Rapid Right Yaw in Helicopters," the loss of tail rotor effectiveness (LTE) is a "critical, low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control." AC 90-95 defined flight characteristics and wind azimuths associated with LTE and stated that the tail rotor vortex ring state occurs during left crosswinds when the relative wind azimuth is from 210° to 330°" and that "winds within this region will result in the development of the vortex ring state of the tail rotor." AC 90-95 also stated that, for all wind azimuths, the loss of ETL is associated with LTE as it "results in increased power demands and additional anti-torque requirements."