A jarring tool having an extensible joint connecting first and second sub ends. A first inner latch piece connects to the upper sub end, and a second outer latch piece connects to the lower sub end. The joint, in a latched position, has the outer latch piece latched to the inner latch piece and the inner and outer latch pieces restrained from unlatching by a stationary restraining collar. Under tensile force the joint unlatches into an unlatched position by the outer latch piece pulling the inner latch piece through the restraining collar into a position where the inner and outer latch pieces are free to separate. An impact force is generated from the tensile force when the joint unlatches and reaches a maximum extension.