Small Engines (Lawn Mowers, etc.): B&S Backfire or starting, technical reason, spark plug

Question
Lawnmower manufactures have recommended for years to disconnect the plug before manually turning the blade by hand. An individual I know claims he clogged the mower and immediately turned it on its side to clean it. When he turned the blade slightly it started and cut nearly all fingers off both hands. Is it possible the engine, with deadman switch and brake, fired or backfired. He may have turned the blade backwards slightly? It was still a very hot engine

Answer
Dear Tom,

Yes, there is a technical reason why you should never service a lawnmower with the spark plug connected.   Engines sometimes utilize a a stop contact to ensure  the spark plug is grounded and also to halt the engine.  A stop contact should guarantee that the engine doesn't accidentally start.   To ensure you're safety the lead should be disconnected from the plug. Also  make sure that you keep the lead separated from the plug during maintenance.  As your friend found out, the engine can sometimes start when the crankshaft is manually turned.