We know that Windows 8.1 would be getting the Start Button back on the desktop and the Start Screen thumbnail from bottom left corner would also be replaced with the Start icon when mouse hovered but, we haven’t seen the Star Button in action. Microsoft demoed the Windows 8.1 Start Button during the TechEd conference along with other features.
Windows 8.1 Preview on Microsoft Surface

The video below shows the Windows 8.1 Start Screen, the ‘All Apps’ section along with the working of Start Button from moving to desktop to Start Screen.
You can also see the option which would show you the ‘All Apps’ screen when you press the Start Button from desktop and Start Screen with tiles is not showed by default.
These options along dozens more features would be available in Windows 8.1 Preview which is scheduled to be launched as an update to Windows 8 on June 26.