Piperpoy,
I worked for an engine overhaul shop for many years. Shock cooling damage is real, and occurs quickly with careless engine operation. As has been said, 1 inch of manifold per minute, or thousand feet maximum rate to reduce power. Yes, the engines will stand more abuse, but not forever, and it only takes once to badly crack a cylinder or 6. Glider tow and paradrop ops usually have a good grasp of these things.
Think of it this way: You have a cast part with many intricate features, in particular variances in the thickness of the casting where there are fins, heavy areas, and thinner areas (between exhaust and intake ports) These all reach their intended operating temperature during normal cruise, and run hotter during a climb, where the engine is working harder, and airspeed (= cooling) is less. During the time available for the engine to heat up after start, these temperature even out. Normal operation keeps them normal.
You're cruising along, and decide to go down. You carelessly snap the throttle closed. The generation of heat pretty well stops, as the engine is no longer developing power. The cooling, however remains the same, 'cause you did not slow down first. So the effect of the cooling is unchanged, but no heat is being generated to balance it. The areas of the cylinder subject to the greatest effect of cooling, will cool very quickly. Other thicker or more shielded areas take longer to cool. Now you have vastly differing temps in the adjacent areas of the same casting, with some shrinking much faster than others (we agree that cooling aluminum causes it to shrink right?). If a whole bunch of crystals of cast aluminum shrink at different rates, some are going to pull away from others, and you get a crack.
I have seen lots of these cracks, and with some cylinders, you just know where to look. You can tell how well a pilot treated that engine. If you must pull power off quickly, pull the nose up lots first, and slow the plane down. At least the slower plane will not cool the engine so fast, and reduce the risk of damage.