EE & CO-WORKER WERE REMOVING OLD TRANSITE SIDINGOFF OF THE PRECIPITATOR BLDG. USING A MAN-LIFT. EE WAS HOLDING A 4'X 8'PIECE OF SIDING WHILE CO-WORKER WAS BACKING OUT LAST SCREW HOLDING THE SIDING. THE SIDING SLIPPED WHEN IT BECAME FREE FROM THE BLDG. & IT DROPPED ABRUPTLY. EE GRABBEDIT & HELD IT FOR AWHILE. HE FINALLY LET IT DROP WHEN HE FELT A SHARP PAIN IN STOMACH AREA.