Usually, the cylinders that get heavily rusted are the ones that have sat for years with one or both valves open. The humidity in the air that cycles in and out due to temperature changes rusts the exposed cylinder walls above the piston. What you're doing by rocking the engine back and forth is slowly dislodging the rust in several inches of cylinder walls. It's a slow process!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!