The helicopter landed hard and rolled over while practicing 180-degree autorotations. The student entered the autorotation and applied excessive collective during the turn. The rpm decreased and the student lowered the nose of the helicopter in order to increase airspeed; consequently, the helicopter was in a low rpm and high airspeed condition as it approached the landing area. The student rolled on the throttle but the descent continued. The CFI then took control of the helicopter but ground impact occurred before he could arrest the descent.