THE AIRLINE TRANSPORT RATED PLT WAS ON A LOW LEVEL, AEROBATIC WAIVER, COMPETENCY CHECK FLT. HE WAS SCHEDULED TO DEMONSTRATE A LOOP, AN AILERON ROLL, A SPLIT-S, AN IMMELMANN, A BARREL ROLL & A 4-POINT HESITATION ROLL. THE MINIMUM ALT FOR THESE MANEUVERS WAS TO BE 500 FT AGL. FOR THE MOST PART, WITNESSES AGREED THE 1ST 5 MANEUVERS WERE PERFORMED WELL, WITH MINOR EXCEPTIONS. HOWEVER, DURING THE FINAL MANEUVER, A 4-POINT ROLL TO THE LEFT, THE ACFT CRASHED. MOST OF THE WITNESSES BELIEVED THE PLT STARTED THE MANEUVER BELOW 500 FT AGL. REPORTEDLY, AT THE 1ST POINT OF THE ROLL, THE NOSE HAD BEGUN TO DROP. AT THE 2ND POINT (INVERTED), THE NOSE HAD DROPPED THRU THE HORIZON TO PERHAPS 10 DEG LOW. ONE EXPERIENCED PLT REPORTED THE ACFT DISHED OUT FROM ABOUT THE 3RD POINT. SUBSEQUENTLY, THE PLANE IMPACTED IN A SOMEWHAT NOSE LOW, RIGHT WING LOW ATTITUDE. THERE WERE INDICATIONS THAT UP ELEVATOR & RIGHT RUDDER WERE APPLIED JUST BEFORE IMPACT. THE PLT HAD APRX 500 HRS SINGLE ENGINE EXPERIENCE & RECENT PRACTICE SESSIONS IN AEROBATIC FLT.