The pilot stated that he was hand-propping the airplane, as it was not equipped with an electric starter. A passenger, who had flown with him twice since the previous fall, was sitting in the front seat. The pilot instructed the passenger to hold the heel brakes with her toes, and initially verified her ability to do so by pushing on the propeller hub. He then returned to the cockpit area and explained the application of full-aft elevator stick to prevent propeller airflow from raising the tail, and the use of the throttle, which he opened 1/8 to 1/4 inch for the start. The pilot also explained the use of the magnetos, had the passenger turn the magneto switch through the "Both" and "Off" positions twice, and advised her that if anything went wrong, to switch the magnetos off. After priming the engine, the pilot called out, "brakes - set," and again confirmed them by attempting to push the airplane from the propeller hub, then called out, "stick - back," and confirmed elevator movement. The pilot threw the propeller, and on the third attempt, the engine caught. As the pilot walked around the wingtip to get into the airplane, the airplane began rolling. He called out for the passenger to turn off the magnetos; however, the airplane rolled into a parked airplane. The pilot did not note any mechanical anomalies with the airplane, nor did he indicate the use of any other airplane securing devices such as chocks or a tail tie-down.