The pilot stated that during approach he noticed the trim was still on. He reached down to turn it off by feel while he was visually focused outside the helicopter and concentrating on the approach. The pilot reported that the trim knob was stuck in the "up" position and he applied increasing pressure until it moved. He said that while focused on the approach, he "blindly checked the status of the trim knob," and that he "unintentionally and without explanation pulled the mixture control up." The pilot reported that the engine instantly turned off and the low rotor RPM system activated, followed by the pilot entering an autorotation. The engine stopped at a height of approximately 50 feet above ground level (agl) and that the helicopter touched down in the dirt and rolled onto the pilot's side. The pilot reported no mechanical failures or malfunctions prior to the accident.