There is a button in some 401(k) websites that says something like "convert to Roth."
It is usually three menus deep, next to nothing that explains it, and if you click it you get a warning screen with the tone of a legal document written by someone who would rather you did not.
That button is an in-plan Roth rollover, and for a meaningful number of people it is the single most valuable feature their retirement plan offers.
It is also the mechanism behind the mega backdoor Roth, which people discuss in hushed tones online as though it were a secret. It is not a secret. It is a menu option that half of plans support and almost nobody uses.
Here is what it does, when it is brilliant, and when it is a mistake.