To explain better:
this-is-my-product-pr-54.html
this-is-my-product-pr-54.html?js=Y
this-is-my-product-print-pr-54.html
this-is-my-product-print-pr-54.html?js=Y
Okay, the 4 above URLs, all go to the same page. The 1st url is considered the *primary* url, has the least fluff in the URL. There are uses for all 4 URL versions though, so each url is active in X-Cart. To avoid duplicate content issues, whenever a bot is detected, only the 1st URL is displayed anywhere to the bot. If the bot tries to visit urls 2, 3 or 4, it gets redirected to url 1. Any urls the bot sees on the page that would normally link to 1,2,3,4, will all instead just display url 1.
So the point is to strip out the known extra parameters that are useless.
Some others incorrectly call this cloaking, but it's not. You can read about cloaking here:
http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66355
The reason why this method works, and has worked for 2-3 years now with no ill effects on Google rankings, is because the actual page content is the same for all variations of a link. So the only "change" when Google visits, is reducing the multiple URLs, to a single URL. Google is smart enough to see that every page has the same content, and so it has not penalized any websites running XCSEO, that I've ever seen.
What's actually interesting, is that I've had users report a definite increase in page rank, when leaving another SEO module, to use XCSEO Basic.
Just a note, XCSEO Pro handles it's urls in a different manner, and does not react to search engines in any way. It's a different approach from XCSEO Basic, and I don't want anyone confusing the two.
Cheers