AFTER TAKEOFF, THE PLT OF THE WEIGHT-SHIFT CONTROLLED, ULTRALIGHT VEHICLE ENTERED A STEEP CLIMB, THEN BEGAN A SERIES OF MANEUVERS WHICH INCLUDED PAST VERTICAL WINGOVERS. REPORTEDLY, ON HIS LAST MANEUVER, HE PERFORMED A WHIP STALL. A GROUND WITNESS STATED THAT DURING THE RECOVERY, HE 'PULLED IN TIGHT TO GET AN IMPRESSIVE DIVE.' A PLT WITNESS & 2 FAA INSPECTORS REPORTED THAT WHEN THE NOSE DROPPED, IT WENT PAST A VERTICAL POSITION. THE PLT WITNESS STATED THAT DURING THE RECOVERY, THE PLT HELD THE NOSE IN APRX A 60 DEG DIVE FOR 3 TO 4 SEC. THE FAA INSPECTORS REPORTED THAT DURING THE PULL-OUT, THE WINGS FAILED. AN EXAM REVEALED THAT THE LEFT & RIGHT INBOARD COMPRESSION STRUTS HAD BUCKLED FROM POSITIVE LOADING. THE PLT HAD MODIFIED THE ULTRALIGHT BY INSTALLING A NON-STANDARD ENG-PROP REDUCTION DRIVE SYS, BY CHANGING THE PITCH TRIM, BY ADDING A LOWER TAIL BOOM BRACE & BY ADDING A 10 LB WT TO THE NOSE TO INCREASE THE FORWARD SPEED IN A DIVE.