The pilot was commencing a practice flight in preparation for a 600 kilometre soaring competition task. The weather was fine with a north-easterly wind of 15-20 knots and a temperature of approximately 40 degrees Celsius. Following line-up the pilot was delayed for approximately fifteen minutes before his aero-tow became available. He then advised the tug pilot that a climb speed of 65 knots was acceptable. The subsequent tow initially appeared normal although observers at the airfield noticed some apparent roll instability. At approximately 1000 feet above ground level (agl) the tug pilot detected thermal activity and began a left orbit to regain the area of lift. With the turn almost completed the right wing was violently "kicked-up" as the aircraft re-entered the area of lift. The tug pilot corrected the upset checked that the glider was not affected and continued the left turn. Approximately five seconds after the upset at about 1300-1500 feet agl the glider released. Observers reported that the glider had banked steeply to the right while apparently still under tow. Following the tow release the angle of bank increased past the vertical the nose dropped steeply and the glider entered a spiral dive from which it did not recover. It struck the ground in an almost vertical nose-down attitude. The investigation was unable to find evidence of any pre-existing structural or system defect. Pilot medical evidence is inconclusive. The reason the aircraft entered an abnormal flight manoeuvre at an altitude from which recovery should have been possible could not be determined.