This is a function that gets called via `init_download_links()`. It gets triggered whenever someone clicks one of the Firefox download button links (`'.download-link'`). Looking at the comments it seems to contain extra logic for IE, that ensures the download prompt appears when the user gets redirected to the download page. 

You should be able to test this on any version of IE - without `trigger_ie_download` there should be no download prompt appearing when you click the link.