Here is how it works. The baseline is the 2008 election. Obama carried the popular vote by 7.2%. The model is actually incredibly accurate in predicting election results. If you assume as most polls today show that Romney wins the popular vote by a slim margin say 3%. You plug in the change amount. In this case the swing from 2008 was 10.2%. According to the widget if Romney wins the popular vote by 3% he will win the election with 285 electoral votes.
A 5% popular vote victory delivers PA to the GOP and Romney wins 316 elecorals.
If the final popular vote is about 50/50 or a slim sub 2% margin for either person, The election would be Obama 272 - Romney 266. Romney would need an outlier like NH or IA to go his way to win.