The helicopter was on a powerline patrol flight with three occupants on board. While proceeding along the powerline with a tailwind, the pilot started to climb over a hill. He stated to the passengers that he was unable to out climb the hill, then circled to the right and gained more airspeed to top the hill. After topping the hill, he started down the other side while to the left and about 10 ft above the powerline with an airspeed of 0 to 20 kts. About 1/4 mile down the hill and at 200 ft lower elevation, the helicopter approached another line that crossed over the line that was being patrolled. At about that time, the helicopter entered a steep turn over the line being patrolled and below the other. The helicopter avoided the lines, but crashed. The transmission sheared during impact, moved forward, and caused fatal injury to the pilot. The passengers suspected a loss of power, but no precrash failure was found. AC61-13B warns of possible "settling with power" in steep power approaches near zero airspeed.