BOTH OCCUPANTS WERE RATED PLTS. THE PLT IN THE RIGHT FRONT SEAT OF THE ACFT HAD APPLIED FOR MEMBERSHIP WITH THE ORGANIZATION THAT OWNED THE PLANE. HE MAY HAVE BEEN RECEIVING INFORMAL INSTRUCTION IN THE OPERATION OF THE ACFT. DURING FLT, THE ACFT WAS OBSERVED IN MANEUVERS WHICH WERE DESCRIBED AS STALLS. AN AIRLINE PLT WHO OBSERVED THE STALLS DESCRIBED THEM AS VIOLENT. AS THE ACFT'S NOSE CAME THRU THE HORIZON, IT REPORTEDLY DROPPED MORE THAN 45 DEG & THE ACFT ENTERED A RAPID LEFT BANK. ANOTHER WITNESS WHO SAW THE ACCIDENT, SAID THE ACFT WENT INTO A 3 TURN SPIN. WHEN THE PLT WAS RECOVERING, THE WITNESS HEARD A 'POP' & SAW SOMETHING TRAIL FROM THE ACFT. ALSO, HE HEARD THE ENG ACCELERATE DURING THE OCCURRENCE. AFTER THE ACFT CRASHED, AN EXAM OF THE CRASH AREA REVEALED THE ACFT BROKE UP IN FLT. MAJOR COMPONENTS THAT SEPARATED INCLUDED A 6 FT OUTBOARD SECTION OF THE LEFT WING, RIGHT AILERON & THE RIGHT DOOR. ALL SEPARATED METAL STRUCTURE DISPLAYED BENDING, TEARING & OVERLOAD FAILURE INDICATIONS.