When you double-click on the icon or using a shortcut key (Windows key + F11), it will resize the currently active window so the title bar is actually above the screen boundaries, thus saving those extra pixels that you don't want to waste.
When you double-click on it again it or using shortcut key (Windows key+F11) will bring back the window to its original size and position.
However for best performance you should set up your taskbar as "auto-hide". If you don't, the lower part of the window will actually be behind the taskbar. You can download the utility from code.google.com