The propeller impacted the individual preparing to hand-prop an engine. The pilot stated that the cranking power of the airplane's battery was diminished because the airplane had been parked outside for an extended time in below freezing temperatures. The pilot was not able to get the engine started using the electric starter. The pilot reported that an acquaintance suggested hand-propping the engine, to which the pilot agreed. The pilot reported that when the accident occurred he was seated in the airplane operating the engine controls. The pilot stated that the magnetos were off and the mixture lever was positioned at idle-cutoff. The pilot reported that as the individual positioned the propeller for the maximum leverage it "snapped [back]" due to cylinder compression. The propeller impact fractured the individual's skull.