The flight instructor was having the student practice landing approaches. With an airspeed of 60 to 65 knots, the student initiated a left base turn, lowering the collective, and adding aft cyclic for a normal approach to an open field. During the turn, the instructor noted that the helicopter was descending faster than anticipated, and that "the collective was too far down, the cyclic was too far back, and [the student] had a tight hold on both controls." The instructor attempted to regain control but could not move the collective or cyclic due to the student's grip on the controls. The instructor said that no words were spoken as he struggled with the student for control of the helicopter for a period of 3 to 4 seconds. The helicopter landed hard, rolled onto its left side, and instantly caught on fire. Both occupants exited the right door. The helicopter was destroyed by fire.