Chrome Extension Developer

I need an Chrome extension developer to tell me how a certain Chrome add-on works. The add-on is https://chrome.google.com/webstore/detail/pubmed-impact-factor/npblmhpjbopmmaadpmheopjelggjnogh?authuser=2 and I need to know where the IF: data is coming from. The data that is added is right after the journal's name and starts with (IF: