Regular Google search helps you find a set of documents that is most relevant to your tasks. Accessible Search goes one step further by helping you find the most accessible pages in that result set.
After doing some searching on it I found that it is a very different index. It matches a normal Google search in the since that there are some of the same sites on the first page of results but they are in a different order. Sites that are mostly text seem to rise higher. Very text heavy sites like wikipedia seem to rank higher. The algo seems to integrate accessibility into its algo for this service. I thought maybe it was more like a filter but it seems to be a lot more than that.
One thing I did see that confused me at first was a lot of flash sites ranking high for some terms. When I did a view source on these sites I noticed one had a noframes tag with lots of good text. I went to another one and it did not. It did have a good title and description. It looks like this algo still uses good titles and anchor text to help a site rank. This seems to show me Google will penalize you for adding things to your site that are not accessible but will not penalize you for having nothing.
Oh yeah and a word to Google Visually Challenged is offensive to blind people. We are blind not challanged.