If you trim out your airplane, it will maintain straight and level flight. Now point your airplane at an object in the distance—say a mountain or building. If there is no wind, eventually you’ll arrive at the object you are pointed at. Now suppose you fly into an area with a crosswind. The entire block of air your are flying in is moving to one side or the other. You haven’t changed anything about how the airplane is oriented but you won’t arrive at your target.

Now, instead of a crosswind, assume you fly into an area of lower pressure. The altimeter doesn’t change but the airplane is now closer to the ground because column of air beneath it is shorter. Unless you get a new altimeter setting you are at risk of flying into the ground (assuming you can’t see it).

By the way, this is a real phenomenon that you need to look out for—not just some theoretical idea for the test. I frequently fly from KSBP to KSMO and when approaching KSBA the new altimeter setting often changes my indicated altitude by 300-400′.

Your reply that “A trimmed out airplane would maintain the same MSL” makes the all the rest of the statements make sense, but why exactly does the airplane “instinctively” hold the same indicated MSL ?

I have a glimmer of understanding here, but could use some reinforcement …

BTW, what happens if you put the altimeter in a refrigerator ?

When you are flying, the altimeter setting you are given has been corrected for non-standard pressure but not non-standard temperature. So if you fly from an area with standard temperature into an area of non-standard temperature, the altimeter setting will not correctly reflect your height above the ground. When flying from the first area to the second, the pilot would not have made any adjustments to trim to maintain his indicated altitude. He would still show the same altitude that he started with. However, the column of air over which he is now flying is denser than the before—because cold air is denser than warm air—hence he is not as far above the ground as he was originally. He thinks that his AGL and MSL are the same, but the AGL is lower than the MSL. In order to maintain the same AGL he would need to climb to a higher MSL. That’s why we say, “HIGH TO LOW LOOK OUT BELOW”.

The effect is much smaller than with non-standard pressure so we don’t normally have to deal with it—but pilots in the Artic do.