Enter your search term, choose your search target -- Google, Amazon, YouTube or Twitter -- and Split.ly will show the results in its left-hand pane. Click on any result and it will appear on the right, taking up most of the screen.
The design is arguably more useful with Google than the other sites (you can normally pick the YouTube or Amazon result you want from a standard search screen, and Twitter already has a multi-pane approach). Split.ly supports Google Instant, so altering your search in the left panel will dynamically change the results.
I can't imagine using Split.ly as my main search, but if you want to quickly go through several results, it's less hassle than clicking back and forth or opening new tabs. Google's preview feature has a similar approach, but only shows you a thumbnail of each site, rather than its current content. (If, like me, you find the previews disorienting when you scroll through results, it's easy to switch them off.) Thanks Chris Z for the tip!