And I’m not going to do it. Instead, take a look at Data Domain’s nifty pdf whitepaper starting on page 12 (or Sec1:11).
The first 11 pages are why it is wonderful. On page 12 they start getting into the how. I have a few quibbles but overall it is well done.
Also, please note that DD doesn’t claim 25x. They claim 20x over time. I use 25x to help the search engines and people referred to the site find what they want.