EMPLOYEE WAS OPERATING A ""LARGE PACK"" PACKAGING LINE WHEN AN EMPTY 20# BAG WAS ATTACHED TO THE END OF A FULL ONE. HE ATTEMPTED TO PULL THE EMPTY BAG OFF BEFORE IT ENTERED A BAG SEALING UNIT. H E OVERREACHED WHEN GRABBING THE BAG AND HIT HIS LEFT THUMB CAUGHT IN THE BELT AND PULLEY ASSEMBLY THAT GUIDES THE BAGS INTO THE SEALER.