Following a local banner towing flight, the pilot returned to the airport to drop the banner. With the flaps extended 10 degrees, the pilot maneuvered the airplane about 200 feet above the runway and slowed to an airspeed of 60 knots. After releasing the banner, the pilot added full engine power and felt a “surge/deceleration.” As the pilot reached down and pulled the banner release handle a second time, thinking the banner had not released, the airplane’s left wing and nosed dropped. The pilot was able to recover to wings level; however, before the descent could be arrested, the airplane impacted the ground and nosed over. The airplane’s left wing and fuselage sustained substantial damage during the impact. A postaccident examination of the airplane revealed no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.