As you can see in the screenshot , prefetch is switched ON. That means when you press stumble it and you browse your current stumble, the stumbleupon toolbar loads the pages that it’s added to the list that will load when you press stumble next. That means, by the time you press the next stumble the page has already been cached and stored. If you use MM3 it might be saved already on your pc and as soon as you click it loads. By changing this setting to false it means that it wont load the future stumbles and wont cache them. That is those stumbles in the stumble list. Every time you push stumble your toolbar fetches a list of upcoming sites. Every few times it fetches a new list.