15.4Why F → Fm → C works so beautifully

F:

F A C

Fm:

F A♭ C

C:

C E G

Only one note changes at first:

A → A♭

Then A♭ resolves naturally:

A♭ → G

So the important inner line is:

A → A♭ → G

This is exactly the kind of semitone voice leading we've been emphasizing.