It is fairly simple, you install the bookmarklet, click it when on the Google results, and the Google+ results will change to either Twitter, Facebook or another social network.
The campaign is named Focus on the User and they write:
How much better would social search be if Google surfaced results from all across the web? The results speak for themselves. We created a tool that uses Google’s own relevance measure-the ranking of their organic search results-to determine what social content should appear in the areas where Google+ results are currently hardcoded.
All of the information in this demo comes from Google itself, and all of the ranking decisions are made by Google's own algorithms. No other services or APIs are accessed.
Here is what it does:
Pretty clear? It does more but those are to examples, many more examples over here.
Danny Sullivan has a great write up over here and here is a video demo:
Forum discussion at WebmasterWorld.