The pilot reported that he was participating in an ash spreading ceremony and planned to do a low pass over the runway. As the airplane approached 50 feet above ground level and 70 knots, the pilot reported that the bag containing the ashes began to break and he "momentarily" let go of the flight controls. Subsequently, the airplane descended, impacted the runway, and bounced back into the air. During the bounce, the pilot regained control and landed straight ahead, but the left main landing gear collapsed on the touchdown. The airplane veered off the runway to the left and skidded to a stop. A postaccident examination of the airframe revealed substantial damage to the firewall. The pilot reported that there were no mechanical malfunctions or failures with the airplane that would have precluded normal operation.