Per document:
> -f, --force
> When git fetch is used with <rbranch>:<lbranch> refspec, it refuses
> to update the local branch <lbranch> unless the remote branch
> <rbranch> it fetches is a descendant of <lbranch>. This option
> overrides that check.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.