You can use BG Double Adsense the same way you use the other Joomla! Adsense modules. IE - put your Google Adsense code into the module and display it the module position. BUT.... if you use a fixed width on your website (which most of us seem to do), BG Adsense can display extra Google Adsense "windows" alongside your normal body content, if the user's browser window is big enough.
BG Double Adsense works by injecting Javascript into your page. The Javascript compares the actual size of the browser window and then compares it to the size of your websites' body tags. It then decides if there is enough room between the body content and the edge of the browser window to display the Google Adsense ad you configured. If there is room for the ad to be fully displayed, a div tag is injected into the HTML, and the Google Adsense code is activated inside the new div.