Creating a transparent backdrop for a website

I want to create the effect these guys have on their site whereby you click an example video link and the screen goes transparent black and the video comes on: http://soloingtechniques.com/SoloPages/examples.html

I know they use the JWplayer http://www.jeroenwijering.com but I can't seem to work out how you create the background which in turn loads the video.

Does anyone know how this is created?

Cheers

jw player is contained in an html file. I have no idea how to make the background of a web page transparent using either jscript or HTML, but i imagine that would be a question for one of the many HTML forums on the web.... :)