when a 4 stroke is lean and "detonates" its like a car backfiring for a split second the engine tries to run backwards thats just enough to spin off the propnut /spinner and sling the prop off the shaft it usually doesnt damage any thing unless flesh is in the way but running your 4 stroke any way than on the rich side of peak idle for extended periods of time will do damage to the engine
P.S. this is why it is NEVER wise to stand anywhere near the prop arc I believe this is the most broken safety rule in RC.