You start off installing it like any other program. Then you set it as your system’s default browser (so there is no running process). Then you might click a link a document you downloaded and a little choosy window would pop-up, similar to the OS X window switcher, with a list of all of the known and enabled browsers you have on your system and you pick one and off you go. How is this different than Open With? It’s browser-selection only, faster and it looks excellent. Here’s the problem. It’s only the Mac, Windows is left out.
I can see no reason why a choosy-like app can’t be made for Windows (and I can’t see why it can’t look similar either). I think it’d be similar to launchy actually. Launchy is a silently running application that only comes up when needed (in it’s case, alt+spacebar) and choosy only comes up when you need to open a link.