What this tool does, precisely

It does two things. It replaces words it recognises with a synonym from a curated list, and it rewrites around forty wordy stock phrases — in order to becomes to, due to the fact that becomes because. That is the whole mechanism. There is no language model behind it and no server call; the entire dictionary lives in the page you are reading.

That design has a real advantage. It is instant, it is free forever, nothing you paste leaves your browser, and the changes are completely predictable. It also has an obvious limit: it does not understand your sentence, so it cannot tell that a particular synonym is wrong in context.

Where synonym swapping goes wrong

English synonyms are almost never exact. Cheap and inexpensive point at the same price and imply very different things about the product. Swap enough words without reading the result and you get prose that is technically English and unmistakably machine-made — the effect people call word salad.

The rewriter leaves proper nouns alone and skips words in the middle of a sentence that begin with a capital, which protects most names and brands. It cannot protect your technical vocabulary, though. If you write about a specific field, check that its terms survived intact.

The honest use for a rewriter

Rewriting to escape a plagiarism checker does not work and is not worth your time; detectors have handled synonym substitution for years, and readers notice long before the software does. What a rewriter is genuinely good for is unsticking yourself: seeing a sentence phrased differently is often enough to show you the version you actually wanted, which is usually neither the original nor the output.

Treat the highlighted result as a rough draft with suggestions, not as publishable copy. Then run it through a spell checker before it goes anywhere.