When dealing with a constant speed prop, it is best to avoid being "oversquare", meaning manifold pressure is higher than acceptable for the current RPM setting. That is why you usually move the throttle first before reducing engine RPMs.
Piston engines are much happier when turning at their natural RPM, but the constant speed prop robs them of the ability to do so (to a degree...you will get to a point where, no matter how the govenor in the prop hub is set, the engine just can't maintain the RPM setting). You will find that, without a computer controlling things, you can make the engine unhappy quickly, and detonation is one of the symptoms that you have
It is similar to lugging the engine in an old car with a carburetor and a manual transmission, or downshifting that same car without giving the engine throttle. There are certain parts of the engine's "envelope" (and I use that word loosely, I just invented it
I've never heard it in the context of the engine before) where the combination of fuel to air, engine rpm, and spark advance just weren't designed to go
You don't find that much in modern cars, because the engine control computer is more flexible than any carburetor setting, timing adjustment, etc. to fix those problems for us. Even fuel injected aviation engines are 100% mechanical (unless someone has certified an aviation EFI system while I wasn't paying attention!).
Turbo aircraft engines are even more sensitive than naturally aspirated ones...