According to multiple statements, the seller of the single-seat, amateur-built airplane performed a preflight inspection with the buyer and an evaluation pilot, then he flew a demonstration flight around the traffic pattern at 300 to 400 feet above the ground and made a full-stop landing. The wind was straight down the runway at 10 knots and gusting. The evaluation pilot, who had not previously flown the airplane model, subsequently performed ground checks and taxi practice before taking off and climbing the airplane to about 400 feet. He then turned the airplane onto the crosswind leg and reduced power before turning it onto the downwind leg. After several seconds on the downwind leg, the airplane began to descend. The pilot added full power to arrest the descent; however, the airplane continued descending at full power, dropping almost vertically before impacting the ground in a level attitude, sustaining substantial damage to the wings and fuselage. Terrain elevation about 3 nautical miles upwind of the runway was about 150 feet higher. No preexisting mechanical anomalies were noted with the airplane, and all of the statements indicated that the engine was producing full power during the airplane's descent. A witness reported that possible low-level windshear was forecast for the region. Windshear, which is a difference in wind speed and direction over a relatively short distance, can result in a rapid loss of altitude, even with full engine power applied.