Winsupersite details how the button works:
When you hover over Start button, the button changes color, with a black background and the accent color used on the flag logo. Yes, it looks exactly like the Start Charm, with similar animations. And when you activate the Switcher app switching utility, the Start button will appear at the bottom as the Start tip does today, and has Start text in the box, as it does now. Finally, in the Milestone Preview (MP) build of Windows 8.1, at least, there is no way to turn off the Start button. That could change.
Notably, the site also says you will be able to use your desktop wallpaper as the Start background, as seen in the screenshot below.
A test version of Windows 8.1 will be released to the public in late June, likely at Microsoft's Build Conference.