Easy. If there's residual fuel vapor in a cylinder (sucked in by the last intake stroke as the engine slowed and stopped) and the crank position stopped on the compression stroke just short of where the mag get's triggered for the next power stroke.

A slight nudge pushes the piston "over the top" of the compression stroke, allowing the compressed fuel/air mixture to expand and turn the crank while also triggering the mag to generate a spark.

Best case, you get one power stroke which would be like getting hit once with s sword. Worst case, the engines starts which would be like getting hit with a Cuisinart (plus the airplane could depart the area).

A basic understanding of engines and metallurgy tells me that you can't hurt the engine by pulling on the prop and I seriously doubt you could hurt a prop hub although that might depend on the design. You could bend a blade. But I wouldn't do it regardless because of the risk of damage to ME if the engine were to fire (even one stroke).