The fact that hands can be, or lacquered, to emulate the old traditional process is enough to know about rather than than the full ins and out of how it’s actually achieved.
In the context of my comment and the photos above, then it seems fairly certain Breguet are wiping/brushing a solution over the top highly polished surface, and ignoring the underside entirely.
As you know, heat isn’t selective about where it travels, so the only way to end up with a result like above would be to remove the colour on the underside with Boric Acid (as Roger Smith does when he goes beyond where he wanted).