...smacked into the baseboard with a crash that must have been heard a thousand miles away.
Sound propagates, so if the sound could be heard a 1000 miles, it could be heard anywhere closer. (Or, synecdochally, if that sound in Los Angeles could be heard in Denver, it could be heard in Las Vegas.) The category here is "places where the crash could be heard", and defining that category requires establishing only the farthest boundary.
Note that if the crash could be heard 2000 miles away, this example does a poor job of defining the category. (This particular example is also hyperbole.)
So, when a category is one-sided, one example at the boundary can define the category. So choosing a concrete, vivid instance works well for definition. So, this is a situation where synecdoche is often found.
A simple example illustrates this:
1. Open every day of the year.
2. Open Christmas and Thanksgiving.
3. Open every day of the year, including Christmas and Thanksgiving.
Days of the year, by itself, isn't a one-sided category. But when the issue is days of the year that the store is open, the category becomes one-sided -- of course the store is open on ordinary days, the issue is if it's open on holidays, with Christmas and Thanksgiving being the extreme.
These one-sided categories tend to arise in fiction. For example, if I am describing my main character as brave, the category is "things that do not scare my main character." This is a one-sided category, so I will give extreme "examples."
The following example is from a book exploring differences between the two genders, and in this scene she is furious at him.
Damn him. I'm going to kill him.
No, [he] has a gun and knows how to use it. I know how to apply mascara and get a five-year-old to go to bed. I would be afraid to kill someone.
This has hyper-specificity – 'mascara' instead of 'make-up', a 'five-year-old' instead of 'young children'. And those instances are appearing from nowhere, suggesting figurative language. Even if the reader does not consciously think about the larger category, it must at least subconsciously be formed in the reader's mind.
The category is, roughly, 'things females can do that males cannot.' The point is that those skills are not good for killing someone (or fighting in general). There is another point that females are skilled, just usually in physical fighting. So extremes work well to establish this.
Plus those are really vivid. Changing a diaper was perfect for the category, but I wanted to make the point that women have a lot of skills beyond the cliches.
I used two instances in that example. Is that synecdoche?
More than One Instance