Is polishing free?
On Crossleaf Standard your membership covers it; on a premium model it costs leaves, usually a little less than a translation because there is no source text to read.
Polishing rewrites an existing translation into cleaner prose: shorter sentences, more natural dialogue, less repetition. It is a second pass over the text, so it is not free of work — it is priced exactly the way a translation of the same text would be.
On Crossleaf Standard that means your membership covers it, and it draws on the same monthly allowance as a translation. On a premium model it costs leaves, quoted and charged the same way as a translation, and usually a little less because there is no source language to read.
You polish a whole chapter from the reader's toolbar. It creates a new version rather than replacing the one you have, so you can read both and switch back at any time.