Many small airplanes have impulse magnetos installed which use sensitive spring-loaded coupling to produce a series of sudden rotations and hot sparks during starting. If the magnetos are on, even the slightest manual turn of a prop may be enough to snap the magneto and start the engine, as this unlucky pilot discovered:
The airplane had been having difficulty starting… The battery had just been replaced. I tried to start the engine without success and got out to adjust the prop. I neglected to turn off the magnetos and when I moved the propeller, the engine started. The prop hit me on the head and right elbow. I was hospitalized for 5 days. The incident could have been prevented if I had been certain I turned off the magnetos.
Any pilot hand-turning a prop should make sure the magnetos are in the off position. In addition, hand-propping is never a one-person job. A second person is always needed in the cockpit to apply brakes.