TWO EMPLOYEES WERE INSTALLING 7"" X 16' CABLE ROOF BOLTS USING A HAND-HELD HYDRAULIC TENSIONER TO TIGHTEN THE BOLTS. ONE WAS PICKING UP THE TENSIONER WHEN THE OTHER ENGAGED THE MANUAL HYDRAULIC VALVE CONTROL CAUSING THE TENSIONER TO RETRACT. HIS RIGHT INDEX FINGER WAS CAUGHT BETWEEN THE CABLE SLEEVE AND THE RETRACTABLE HEAD OF THE TENSIONER, RESULTING IN AMPUTATION AT THE 1ST JOINT.