The pilot of the tow plane reported that after takeoff at an altitude of about 50 ft agl, he felt a 'large pull' on the tail of his airplane. He looked back and saw the glider was 'very high and off to my left side.' He then felt slack in the line at which time he thought the glider was moving back into position. At an altitude of 100' to 150' a 'violent jerk' occurred and the tow plane was forced into a nose down attitude. The tow rope was released but the airplane continued in the nose down attitude until it contacted the terrain.