What does --subvert do?

[bound-variable]

Running ruplacer --help prints of --subvert:

Replace all variants of the pattern (snake_case, CamelCase and so on)

Replace all variants with what? The verb replace is transitive, so what are we replacing "the variants of the pattern" with?

The README gives as example:

$ ruplacer --subvert foo_bar spam_eggs
Patching src/foo.txt
-- foo_bar, FooBar, and FOO_BAR!
++ spam_eggs, SpamEggs, and SPAM_EGGS!

So it seems that what's happening is the "pattern" is substituted with the "replacement" in such a way as to maintain letter casing. If so, why not add this sort of clarification in the help? Also, I'm confused about why the option is called subvert? I checked the OED [paywalled] and Collins dictionaries, and I don't see any meaning of subvert that would be applicable here.