If you're talking about spinners, I know how they work.
Most spinners will try to change the words by looking up in their dictionary for words/phrases with similar meanings.
But most of the time the whole article becomes crap. It wouldn't mean anything just because these tools tends to change lots of the words with similar ones.
BTW some spinners do have an option to set the frequency. So it will be like in every 15 words change one word or in 50 words change one word or something like this! If you use these quite intelligently, you could get a good article which is not that bad.