It can figure out what information is most interesting by looking at the load order for the page and then also by analysis of the component assets on the page. For instance the JPGs, PNGs, the HTML itself. So for example in the cases of images the format of the data itself will inform us which bytes are most important (low frequency DCT's are more visually important than high frequency DCT's) or in the case of HTML, sending the <HEAD> section first is more important than the <BODY> or footer. You can find more info like whitepapers and webinars here
http://instartlogic.com/resources/.