GARDINER, N.Y., Aug. 20 - From 5,000 feet up, a parachutist glides gently to earth, destined for a soft, feathery landing in the grassy airfield here at the Blue Sky Ranch, at the base of the Shawangunk Mountains. But suddenly, at about 500 feet, the jumper abruptly begins plummeting in a downward spiral and seems in a free-fall death plunge. At precisely this unnerving moment, though, cheers erupt from a crowd assembled around a small artificial pond below, and the jumper deftly pulls out of the plunge by banking a turn toward the water. In a steep, swooping approach, he...