The one flag --help does not print

--thumb <name> renders one preset’s browser thumbnail into the cache and exits. It is deliberately absent from the roster, because the process that passes it is the player itself: the app re-invokes its own executable one preset at a time to fill the thumbnail cache below (ADR-0230). Like --schema and --check it opens no window, binds no socket and starts no capture client.

It is documented here rather than hidden outright so that a ritmolux seen in a process list with an argument nothing explains is not a mystery. Running it by hand is harmless — it renders the still for one preset of whatever library this launch resolves, writes the cache entry, and says on standard error either what it wrote or that the preset has not changed since last time. A name no library holds is refused and nothing is written.