I honestly do not know what to make of this. A thread at WebmasterWorld named Site Buried in Yahoo Serps: -asdf Syndrome where it shows if you do a search in Yahoo! with and without adding "-asdf" or whatever, it brings up different results. Understand that by adding this negative operator it tells Yahoo! to exclude any pages that have those words on it. So when using an obscure phrase like "asdf" you would assume it would not change SERPs.
When you compare the two, only one site overlaps in the top 10 results. The site is htmlhelp.com which shows in the non -asdf result as number nine and with the -asdf as number two. If you look at Google the order is slightly different, when comparing web design and web design -asdf; but the same results show.
People in the forum believe that this may be a way to see future or past results, after the new algorithm came out. Now, Google did this in the past with the Florida update, with the introduction of the Google Sandbox. Would Yahoo! do the same. I really don't think so. But who knows. :)
Forum discussion at WebmasterWorld.