The helicopter pilot reported that while flying at a low altitude and low airspeed, over tree-covered terrain, he made a steep left turn downwind, and the low rotor rpm horn sounded. He stated that he lowered the collective slightly and rolled on throttle, but the helicopter descended and impacted trees and terrain. The pilot reported that he completed a damage assessment and flew the helicopter to his home airport without further incident. Substantial damage was sustained to the helicopter left aft fuselage frame assembly. The pilot reported that there were no mechanical malfunctions or anomalies prior to or during the flight that would have prevented normal helicopter operation. ADDITIONAL INFORMATION Vortex Ring State (Commonly known as "settling with power") FAA-H-8023-21A identifies, "A vortex ring state may be entered during any maneuver that places the main rotor in a condition of descending in a column of disturbed air and low forward airspeed. Airspeeds that are below translational lift airspeeds are within this region of susceptibility to settling with power aerodynamics (pg. 11-10)." "Some of the situations that are conducive to a settling with power condition are: any hover above ground effect altitude, specifically attempting to hover OGE at altitudes above the hovering ceiling of the helicopter, attempting to hover OGE without maintaining precise altitude control, pinnacle or rooftop helipads when the wind is not aligned with the landing direction, and downwind and steep power approaches in which airspeed is permitted to drop below 10 knots depending on the type of helicopter (pg. 11-10)."