EE AND 2 OTHERS WERE RAISING AND POSITIONING A STEEL BANK BOARD AT A NEW BELT TRANSFER INSTALATION USING 3 COME-ALONGS. ONE SET OF COME-ALONGS WAS DISCONNECTED TO READJUST THE HOOKUP; A SECOND SET SLIPPED OFF THE BANK BOARD, ALLOWING THE BANK BOARD TO TILT AND FALL. THE BANK BOARD PINNED THE INJURED EE AGAINST THE CONVEYOR BELT.