A clever use of history.replaceState and some special characters (to make it less obvious that the state's been altered) - "view source" in Chrome operates on the currently active URL, which after the replaceState has executed is the "not that easily" page.
You can also reproduce the behavior by navigating back, then forwards again - you'll see the "not that easily" page that way, as well (as the "try and view the source" page has been replaced in your history list).
You can also reproduce the behavior by navigating back, then forwards again - you'll see the "not that easily" page that way, as well (as the "try and view the source" page has been replaced in your history list).