On February 23, 1994, about 1050 hours Pacific standard time, the instructor pilot of a Piper PA-28-140, N20BS, was struck by the propeller of the airplane at the Sonoma County Airport, Santa Rosa, California. The airplane was to be operated as a visual flight rules (VFR) instructional flight to Sacramento, California. The airplane, registered to the pilot and operated by Redwood Aviation Enterprises, Santa Rosa, was not damaged. The certificated commercial pilot/certified flight instructor received serious injuries. A student pilot seated in the airplane was not injured. Visual meteorological conditions prevailed. A Federal Aviation Administration (FAA) operations inspector, Oakland Flight Standards District Office, reported that the pilot and student were preparing to depart on their flight. The student attempted to start the airplane; however, the engine was cranking too slowly to start. The instructor exited the airplane to arrange for an electrical "jump" start. The student inside the airplane did not manipulate the controls while the instructor was outside. The instructor turned the propeller to position it for the start. The engine caught and rotated the propeller, striking the instructor in the head and shoulder. Examination of the cockpit controls revealed that the master switch was OFF, mixture and throttle controls at idle, and the magneto switch was ON. The pilot reported that she was unsure whether the propeller was stopped at the top of a compression stroke causing it to rotate suddenly when it was moved, or whether the engine actually fired.