INJURED AND A CO-WORKER HAD FINISHED INSTALLING A ROW OF CABLE TRUSS PIGTAILS USING A TWIN BOOM BOLTER. AFTER CO-WORKER RELEASED THE TRS FROM THE ROOF A PIECE OF SLATE 8"" THICK X 36"" LONG X 24 "" WIDE FELL FROM THE TRS AND STRUCK VICTIM ON THE LEFT HAND. HE WAS STANDING BETWEEN THE RIGHT DRILL HEAD AND THE TRS SLIDE (PINCH POINT) TO POSITION DRILL FOR NEXT ROW.