Okay, okay, I guess you'd like a little more information than that! Each person who signs up with Adsense is given an ID number. When an ad is displayed on a page it is associated with one particular ID.
On an Adsense revenue sharing site the site owner defines a set of conditions under which publisher ID numbers are used other than his or her own. A common example might be to show the ID of the author of an article. Alternately, on a forum, the person who started a thread might have their ID used when the thread is viewed.
Generally the publisher will swap out their own ID only a percentage of the time. A common number is 50%, but it could be anything.
Why Do People Share Revenue?
Well, sometimes when you aren't greedy, it ends up paying off. In this case, when a publisher is able to entice other authors to participate, both of them may end up more profitable.
Often the publisher will have more public awareness and more traffic than the author. At the same time the publisher may find the generation of fresh new content on a variety of subjects to be time consuming or expensive. So, by exchanging traffic for content, both parties may end up winning.
Is This Allowed By Google
Yes. However, you do have to make sure the content published meets the standards put forth by Google. Adsense cannot be displayed on pages containing certain types of content or on pages that break Google's terms of service.
What Are The Dangers
There shouldn't be any dangers, other than violating Google's terms of service, but there is a potential surprise. You have to be sure that you do not click your own ads while viewing a revenue sharing site. Generally, the publisher will be able to not use your publisher ID while you view their site, but of course you will have to be logged in for that to work.
Once you are in a revenue sharing relationship with a site, never view that site without being logged in. Also, check carefully with the publisher to ensure that they do employ this level of protection. It's also quite possible that some smaller revenue sharing sites have tried to implement this but failed to do so due to buggy programming (even if it is quite a simple idea).
Can I Make Money This Way?
Sure, it all depends on keywords and traffic. For example, over at Digital Point, which is a revenue sharing forum, I have a click today that is worth $3.39 on a single page view. Of course, it generally takes months for me to notice a click on Digital Point, but this was a pleasant surprise. Too bad I don't know which thread or post the click originated from -- as I'd be very happy to create content on that particular topic.