DRIVER WAS UNTARPING A LOAD OF BAGGED CONCRETE AT CUSTOMER'S STORE. DRIVER JUMPED DOWN FROM PALLET OF CONCRETE TO FLOOR OF FLAT BED TRAILER, TRIPPED ON TARP AND STUCK OUT HAND TO STOP HIS FALL. HE HIT HIS HAND ON THE FRONT BOARD OF TRAILER. FRACTURED CLOSE METACARPAL / RIGHT HAND.