It's that last read-through before I let Someone Other get a peek at my work. Generally it involves removing all traces of myself from the MS: cutting out little asides that are me, not my characters; removing words and phrases that ring of author rather than authenticity.
Here, too, I ask myself if I'm sure of my facts, usages, and vocabulary. For example, I found I'd used the word "alibi", and although it wasn't in conversation, Elizabethans didn't have the word and it sounds anachronistic on re-examination.
This final read is for tweaking; now that I know how it all works out, I can change a word, add a phrase, or move a passage to create foreshadowing, to highlight an event, or to add a teaser/clue.
When this day is done, the MS will no longer be mine alone. That's a relief, but there's also a modicum of dread.