It finds the width/height of the users desktop, then creates a windowed window of that size, then uses some api commands to get rid of the blue bar. End result is something that looks like fullscreen, but is windowed in the exact dimensions of the users desktop.
If you wanted to use real fullscreen mode, you could just take the width/height from the api calls.
If you wanted to use real fullscreen mode, you could just take the width/height from the api calls.