As with all systems, knowing how the back end works means you might actually have some luck on the front end. The Venture Skills blog goes into how they think that Stumble Upon works, and it goes into some pretty interesting mathematics, and assumptions.

Like all reverse engineering, this may or may not really work.

Every stumbler has an audience score in the old days stumbleupon told you what your score was but have since taken this facility away. The audience score was based on number of fans, number of pages thumbed up, number of pages thumbed down and number of reviews written. The score is what determines how much stumble juice a person carries. The audience score has one other factor stumble history. If a stumbler initially stumbles a site and the site receives a large quantity of thumbs up their audience score increases conversely if they initially stumble a site and it’s thumbed down their audience score goes down. Stumblers who stumble a site after the initial stumble also have changes to their audience score but not to the same extent. Source: Venture Skills Blog

The commentary is also interesting, the best ones are from the Blog Herald, and How to Split an Atom, in that they add to the discussion on how Stumble Upon works, and gives it some nice spin on how to gain more audience share for the web sites themselves.

The idea that they are realistic about the whole process is a good thing, while we randomly stumble things on our way through the Internet, it is also a given that if we stumble the same site over and over again that it will not take in the stumble upon system. Try to stumble this article and you will see what we mean. Too many too quickly from the same site means that they might not take in the longer run. Sites are easy, individual pages can be difficult as the system will simply time out, show a blank screen and provide no feedback.

It would be great if stumble upon provided some feedback on why pages can not be stumbled as easily if they are a repeat, or if one particular article on a web site you liked previously was not taken into the system.

Well worth reading, a bit technical, but then when you are reverse engineering a system, technology and language is what you need to have to understand what they are saying.