At 16:50 PDT, a Bellanca 7KCAB crashed after the pilot lost control during practice aerobatic maneuvers. The pilot bailed out and sustained minor injuries. The pilot said he climbed to 6,000 feet and entered a left upright spin. After two turns, he initiated a forward movement of the stick to recover and found that the stick was jammed in the full aft position. He looked in the rear seat area and discovered that the rear seat back cushion had fallen forward, looped over the stick, and was holding it in place. Following unsuccessful attempts to recover, the pilot abandoned the airplane.