EE WAS ADJUSTING THE BAG DISCHARGER ON THE 787 FORCE FLOW VALVE PACKER TO ACCOMODATE 50 LBS BAGS. AS HE REACHED DOWN HE SLIPPED HIS RIGHT THUMB WENT UNDER THE BAG DISCHARGER AND HIS RIGHT SHOU LDER HIT THE START BUTTON.T HE BAG DISCHARGER CLOSE ON HIS THUMB. THE RELEASE BUTTON WAS PUSHED 3-5 SECONDS LATER. HIS THUMB WAS SEVERLY LACERATED.