Club members that witnessed the accident are in agreement that Bob stall/spun on tow at about 1/2 way up at 1000 feet or so and made no attempt to recover with about 3 turns to impact. The glider back released almost immediately after onset of the stall/spin. Personally I think he reacted to a slow on tow condition by pulling instead of pushing over to cause the spin then experienced sensory overload as the spin progressed. Very sad day at Prescott Soaring. The only serious accident in 30+ plus years of winching.