INJURED, OPERATING A HAUL TRUCK WAS BACKING INTO THE RIGHT SIDE OF #578 SHOVEL TO GET LOADED. THE SHOVEL OPERATOR, WITH THE DIPPER POSITIONED IN THE AIR, ATTEMPTED TO SET THE HOIST BRAKE WITH HIS LEFT HAND. IN THE ATTEMPT TO SET THE HOIST BRAKE, THE BRAKE WAS UN-INTENTIONALLY RELEASED. AS THE SHOVEL OPERATOR RELEASED THE HOIST CONTROL THE DIPPER FREE-FELL STRIKING THE DUMP BOX.