While practicing an autorotation maneuver, the pilot rolled the throttle back on and began a power recovery before reducing the throttle to continue the autorotation. The helicopter's airspeed decreased to approximately 40 knots at 300 feet above ground level (AGL). The pilot attempted to go-around but was unable to increase the rotor RPM (rotations per minute.) The helicopter continued to sink as the flight instructor attempted to increase power and reduce the sink rate. The flight instructor lowered the helicopter's nose in an attempt to increase airspeed. At 50 feet AGL the pilot reported that a gust of wind decreased the helicopter's airspeed further resulting in an increased sink rate. The flight instructor flared the helicopter as it approached the ground. The helicopter contacted the ground and rolled coming to rest on its side.