The pilot reported that, during the approach and while the airplane was about 500 ft above ground level and 81 knots, he "felt the descent rate increase significantly." The pilot increased engine power, but "the high rate of descent continued," and he then increased the engine power further. A slow left roll developed, and he applied full right aileron and full right rudder to arrest the left roll. He also reduced the engine power, and the left roll stopped. The pilot regained control of the airplane, but the airplane's heading was 45° left of the runway heading, and the airplane impacted trees and then terrain. The airplane caught fire, and the pilot and passenger exited through the emergency exit. The airplane sustained substantial damage to the windscreens and fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.