The pilot reported that the initial takeoff roll was normal, but that after a premature takeoff due to wind gusts and thermals the glider controls became ineffective. The pilot attempted to pull the tow line release knob and reported that the tow line “back-released,” resulting in the glider entering a stall. During the impact with terrain, the glider's empennage separated partially, just aft of the fuselage. Examination of the tow line release mechanism and glider systems revealed no mechanical anomalies.