ON 7/27/83, MILD MAST BUMPING OCCURRED DURING A PUSHOVER MANEUVER WHILE A HILLER FH-1180 WAS ON A FLT TO CERTIFY NEW ROTOR BLADES. PRIOR TO THE INCIDENT, THE HELICOPTER WAS ACCELERATED TO 90 MPH. THIS WAS FOLLOWED BY A PULL-UP & DECELERATION TO 80 MPH, THEN THE PUSHOVER WAS INITIATED. THE HELICOPTER REPORTEDLY ROLLED SHARPLY TO THE RIGHT & PITCHEDNOSE DOWN WITH SOME YAW TO THE RIGHT. RECOVERY WAS MADE WITH LEFT CYCLIC; HOWEVER, DIRECTIONAL CONTROL WAS LOST BEFORE RECOVERY & MILD MAST BUMPING. ON 7/28/83, FH-1100, N450FH, A STANDARD PRODUCTION MODEL, WAS FLOWN TO EXPLORE THE PHENOMENON. AFTER 2 PUSHOVERS WITH INCREASING SEVERITY, IN WHICH CONTROL WAS RETAINED, THE PLT ATTEMPTED A REPETITION OF THE PREVIOUS DAY'S MANEUVER. WITNESSES OBSERVERVED THE HELICOPTER ROLL & YAW TO THE RIGHT & PITCH DOWN AS IT BEGAN TO BREAK UP, THEN BURN & CRASH. AN EXAM REVEALED THE MAIN ROTOR BLADES SEPARATED AFTER MAST BUMPING & MAST FAILURE OCCURRED. THERE WAS EVIDENCE THE PLT MIGHT NOT HAVE BEEN CURRENT IN TEETERING ROTOR SYSTEMS & RECOVERY FROM LOW G ROLLS.