THE INSTRUCTOR PILOT WAS DEMONSTRATING COCKPIT AND RUNUP PROCEDURES TO THE STUDENT, WHEN THE HELICOPTER BECAME LIGHT ON THE SKIDS WITH OPERATING RPM & GROUND RESONANCE DEVELOPED. THE INSTRUCTOR BOTTOMED THE PITCH & SHUT OFF THE ENGINE, BUT THE GROUND RESONANCE BECAME WORSE. SUBSEQUENTLY, THE TAIL BOOM WAS CHOPPED OFF BY THE MAIN ROTOR BLADES, A FUEL CELL RUPTURED & A FIRE ERUPTED. ACCORDING TO AN FAA HANDBOOK (AC 61-13B), THE CORRECT PROCEDURE FOR GETTING OUT OF GROUND RESONANCE WAS TO LIFT OFF. AN EXAM REVEALED THE LEFT FRONT SHOCK STRUT WAS LOW.