Caffeine was an indexing change, not a ranking change. Google's post of 9th Jun 2010 announced "the completion of a new web indexing system called Caffeine".
The headline claim: "Caffeine provides 50 percent fresher results for web searches than our last index, and it's the largest collection of web content we've offered."
Google explained the difference. The old index "had several layers, some of which were refreshed at a faster rate than others".
Caffeine analysed the web in small portions and updated the index continuously.
The practical effect for publishers: "you can now find links to relevant content much sooner after it is published than was possible ever before". The Freshness update of 2011 built on it.
