The pilot was approaching to land at the end of a local soaring flight. He commenced final approach at a satisfactory height and speed. The air brakes were selected half extended in order to control the approach. A short time later the aircraft began to slow down and sink more rapidly than the pilot desired so he lowered the nose and attempted to retract the air brakes. He was unable to retract the brakes and the aircraft struck the boundary fence short of the runway. On later consideration of his actions the pilot found that he had been trying to extend the airbrakes rather than retract them. He considered that this was due his anxiety at the high rate of descent that had developed. The initial high rate of descent was probably a result of unexpected sink encountered on approach.