The flight instructor reported that he had given instructions to the student pilot to hover over a grass area on the airport property. The flight instructor stated the helicopter was "hovering 18 inches to 30 inches above the ground and then was lying on its side on the ground". The helicopter sustained substantial damage to the main rotor, tail boom, and tail rotor drive shaft. In a statement to the FAA, the flight instructor said he had given the controls to the student pilot. The student pilot started to over control the helicopter and it went into a pendulum motion. The helicopter came close to the ground and the right skid made contact with the ground, causing a dynamic rollover. The flight instructor reported no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.