EMPLOYEE WAS INSTALLING ROOF BOLTS WITH 2 OTHER MINERS OFF THE SCISSOR DECK TRUCK. HE WAS BEING TASK TRAINED ON THE OPERATION OF THE TRUCK. ANOTHER MINER STARTED TO COLLAR A HOLE IN THE BACK, AND AS HE TURNED THE FEED TO THE JACKLEG, THE MACHINE GOT AWAY FROM HIM. IN HIS EFFORT TO CONTROL THE JACKLEG HE PULLED IT OVER AND IT FELL BEHIND HIM STRIKING BACK OF NECK.