UPON COMPLETION OF HIS ROUTINE DURING AN AIRSHOW, THE PLT STATED TO THE SHOW ANNOUNCER THAT HE WOULD DO A LOW PASS DOWN THE RWY FOLLOWED BY A VERTICAL CLIMB TO A HAMMERHEAD STALL & THEN INTO A LOMCEVAK AEROBATIC MANEUVER. THIS MANEUVER IS ACLIMBING OUTSIDE SNAP ROLL THAT IS PERFORMED IN SUCH A WAY THAT THE ACFT'S UPWARD INERTIA OVERCOMES ITS NORMAL AERODYNAMIC SNAP ROLL TENDENCIES, CAUSING IT TO TUMBLE END-OVER-END BRIEFLY BEFORE IT DETERIORATES INTO A FLAT INVERTED SPIN. ENTRY INTO THE MANEUVER WITH ACFT SUCH AS THE EAGLE IIIS AT AN ALTITUDE OF AT LEAST 1,000 AGL AT SEA LEVEL. THE ENTRY ALTITUDE MUST BE INCREASED TO COMPENSATE FOR THE REDUCTION OF THE ACFT'S VERTICAL PENETRATION WHICH OCCURS WHEN THE MANEUVER IS PERFORMED AT HIGHER ALTITUDES. A VIDEOTAPE OF THE ACCIDENT SHOWED THAT THE PLT INITIATED THE MANEUVER ATA PULL-UP ALTITUDE OF 500-600 FT AGL. THE DENSITY ALTITUDE AT THE ALTITUDE THAT THE PLT ENTERED THE MANEUVER WAS 6,800 FT.