WHILE HOVERING, THE INSTRUCTOR PILOT LOST CONTROL OF THE HELICOPTER BY ALLOWING THE STUDENT PILOT TO OVERCORRECT THE CONTROLS AND BY NOT TAKING IMMEDIATE CORRECTIVE ACTION. AFTER HE RECOGNIZED THE DETERIORATING CONDITION, HE TRIED TO RECOVER A GROUND RESONANCE PROBLEM. SHORTLY AFTERWARD, THE HELICOPTER ROLLED OVER ON ITS SIDE. NO PRIOR MECHANICAL PROBLEM WAS REPORTED.