The pilot of the ultralight vehicle, a student, was in radio contact with his instructor through the use of a portable hand-held UHF transceiver. About 10 minutes after takeoff, the instructor asked him to return to the airstrip for a practice power-off landing. During the approach, when the vehicle was about 1/2 mile from the strip at 600 to 800 feet above ground level, the instructor asked him to reduce the power to idle. The instructor had heard the power reduction to about 50% when he observed the nose of the vehicle pitch down. From his position, the vehicle appeared to flip over, similar to an outside loop, but with no loss of altitude. (The operator believed there might have been a rapid nose down, then nose up pitch which appeared to be a tumble.) Immediately after the vehicle returned to an upright position, the wings folded upward, then the ultralight entered a descent and crashed. No evidence of fatigue or pre-accident malfunction was found. A pathological exam revealed evidence of coronary artery disease and angina. The medical examiner suspected physical impairment.